Add a property to the resolver config to allow for a full resolvergen configuration

[penard-cb]

What happened?

I need to be able to run gqlgen and pass in a template for generating a resolver.

I need to be able to pass in new imports and make significant code additions (maybe even some convenience methods). This will allow my implementing team to follow my guidlines.

What did you expect?

I thought the config would have an option to just replace the whole gotpl file... it doesn't.

Minimal graphql.schema and models to reproduce

type Query {
    resolver: Resolver!
}

type Resolver {
    name: String!
}

schema:
  - "testdata/schema.graphql"

exec:
  filename: testdata/singlefile/out/ignored.go
model:
  filename: testdata/singlefile/out/generated.go
resolver:
  type: CustomResolverType
  layout: follow-schema
  dir: testdata/resolvertemplate/out
  filename_template: "{name}.resolvers.go"
  resolver_template: "testdata/resolvertemplate/customResolverTemplate.gotpl"

models:
  Resolver:
    model: github.com/99designs/gqlgen/plugin/resolvergen/testdata/singlefile/out.Resolver