Adding entity resolver tests in the federation plugin

The tests work by sending `_entities` queries with `representation` variables directly to the mocked server, which will allow us to test the generated end to end.  For context, the format of the entity query is something like:

```
query($representations:[_Any!]!){_entities(representations:$representations){ ...on Hello{secondary} }}
```

And `representations` are the list of federated keys for the entities being resovled, and they look like

```
representations: [{
   "__typename": "Hello",
   "name":       "federated key value 1",
}, {
   "__typename": "Hello",
   "name":       "federated key value 2",
}]
```

The entity resolver tests are in `plugin/federation/federation_entityresolver_test.go` and they rely on `plugin/federation/testdata/entityresolver`.

NOTE: currently there are two regressions from 99designs#1684.
1. The generated code federation plugin creates a self executing function for resolving entities. The issue is that the self executing function specifies the graphql entity the resolver returns and it includes the namespace. So generating federation.go in the same package as the generated models causes compile errors.  See plugin/federation/testdata/entityresolver/federation.gp:70.
2. When there are multiple federated keys on an entity and one is nested, depending on the order in which the entities are resolved there can be exceptions.

Because of the first issue, the generated code has to be manually changed to remove the namespace `generated` from `testdata/entityresolver/generated/federation.go`. Then you should be able to run the tests.  The second test fails because of issue two.

To run the tests:
1. Build the entityresolver testdata
  - From plugin/federation, run `go run github.com/99designs/gqlgen --config testdata/entityresolver/gqlgen.yml`
2. Run the tests with `go test ./...` or similar

plugin/federation/federation_entityresolver_test.go

@@ -0,0 +1,102 @@
+package federation
+
+import (
+	"strings"
+	"testing"
+
+	"github.com/99designs/gqlgen/client"
+	"github.com/99designs/gqlgen/graphql/handler"
+	"github.com/stretchr/testify/require"
+
+	"github.com/99designs/gqlgen/plugin/federation/testdata/entityresolver"
+	entityresolverGenerated "github.com/99designs/gqlgen/plugin/federation/testdata/entityresolver/generated"
+)
+
+func TestEntityResolver(t *testing.T) {
+	c := client.New(handler.NewDefaultServer(
+		entityresolverGenerated.NewExecutableSchema(entityresolverGenerated.Config{
+			Resolvers: &entityresolver.Resolver{}}),
+	))
+
+	t.Run("Hello entities - single federation key", func(t *testing.T) {
+		representations := []map[string]interface{}{
+			{
+				"__typename": "Hello",
+				"name":       "first name - 1",
+			}, {
+				"__typename": "Hello",
+				"name":       "first name - 2",
+			},
+		}
+
+		var resp struct {
+			Entities []struct {
+				Name string `json:"name"`
+			} `json:"_entities"`
+		}
+
+		err := c.Post(
+			entityQuery([]string{
+				"Hello {name}",
+			}),
+			&resp,
+			client.Var("representations", representations),
+		)
+
+		require.NoError(t, err)
+		require.Equal(t, resp.Entities[0].Name, "first name - 1")
+		require.Equal(t, resp.Entities[1].Name, "first name - 2")
+	})
+
+	t.Run("World entity with bar key", func(t *testing.T) {
+		representations := []map[string]interface{}{
+			{
+				"__typename": "World",
+				"hello": map[string]interface{}{
+					"name": "world name - 1",
+				},
+				"foo": "foo 1",
+			}, {
+				"__typename": "World",
+				"hello": map[string]interface{}{
+					"name": "world name - 2",
+				},
+				"foo": "foo 2",
+			},
+		}
+
+		var resp struct {
+			Entities []struct {
+				Foo   string `json:"foo"`
+				Hello struct {
+					Name string `json:"name"`
+				} `json:"hello"`
+			} `json:"_entities"`
+		}
+
+		err := c.Post(
+			entityQuery([]string{
+				"World {foo hello {name}}",
+			}),
+			&resp,
+			client.Var("representations", representations),
+		)
+
+		require.NoError(t, err)
+		require.Equal(t, resp.Entities[0].Foo, "foo 1")
+		require.Equal(t, resp.Entities[0].Hello.Name, "world name - 1")
+		require.Equal(t, resp.Entities[1].Foo, "foo 2")
+		require.Equal(t, resp.Entities[1].Hello.Name, "world name - 2")
+	})
+}
+
+func entityQuery(queries []string) string {
+	// Wha we want!
+	//query($representations:[_Any!]!){_entities(representations:$representations){ ...on Hello{secondary} }}
+	entityQueries := make([]string, len(queries))
+	for i, query := range queries {
+		entityQueries[i] = " ... on " + query
+	}
+
+	return "query($representations:[_Any!]!){_entities(representations:$representations){" + strings.Join(entityQueries, "") + "}}"
+}

plugin/federation/readme.md

@@ -0,0 +1,17 @@
+# Federation plugin
+
+Add support for graphql federation in your graphql Go server!
+
+TODO(miguel): add details.
+
+# Tests
+There are several different tests. Some will process the configuration file directly.  You can see those in the `federation_test.go`.  There are also tests for entity resolvers, which will simulate requests from a federation server like Apollo Federation.
+
+Running entity resolver tests.
+1. Go to `plugin/federation`
+2. Run the command `go run github.com/99designs/gqlgen --config testdata/entityresolver/gqlgen.yml`
+3. Run the tests with `go test ./...`.
+
+# Architecture
+
+TODO(miguel): add details.

plugin/federation/testdata/entityresolver/entity.resolvers.go

@@ -0,0 +1,30 @@
+package entityresolver
+
+// This file will be automatically regenerated based on the schema, any resolver implementations
+// will be copied through when generating and any unknown code will be moved to the end.
+
+import (
+	"context"
+
+	"github.com/99designs/gqlgen/plugin/federation/testdata/entityresolver/generated"
+)
+
+func (r *entityResolver) FindHelloByName(ctx context.Context, name string) (*generated.Hello, error) {
+	return &generated.Hello{
+		Name: name,
+	}, nil
+}
+
+func (r *entityResolver) FindWorldByHelloNameAndFoo(ctx context.Context, helloName string, foo string) (*generated.World, error) {
+	return &generated.World{
+		Hello: &generated.Hello{
+			Name: helloName,
+		},
+		Foo: foo,
+	}, nil
+}
+
+// Entity returns generated.EntityResolver implementation.
+func (r *Resolver) Entity() generated.EntityResolver { return &entityResolver{r} }
+
+type entityResolver struct{ *Resolver }