[proposal] Add an extension interceptor for the Root Field (RootFieldInterceptor)

[CoreyWinkelmannPP]

When implementing an extension for gqlgen to wire in monitoring for New Relic I noticed there is no way to segment out the Root Field execution separate from the rest of the fields. My hope is to implement a new Intercept interface to accomplish this capability. Something like the following.

	RootFieldInterceptor interface {
		InterceptRootField(ctx context.Context, next RootResolver) Marshaler
	}

This would wrap the marshaler logic within the object.gotpl file when $object.Root. Here is the example within the $field.IsConcurrent section.

        case "{{$field.Name}}":
            {{- if $field.IsConcurrent }}
                field := field

                innerFunc := func(ctx context.Context) (res graphql.Marshaler) {
                    defer func() {
                        if r := recover(); r != nil {
                            ec.Error(ctx, ec.Recover(ctx, r))
                        }
                    }()
                    res = ec._{{$object.Name}}_{{$field.Name}}(ctx, field{{if not $object.Root}}, obj{{end}})
                    {{- if $field.TypeReference.GQL.NonNull }}
                        if res == graphql.Null {
                            {{- if $object.IsConcurrent }}
                                atomic.AddUint32(&invalids, 1)
                            {{- else }}
                                invalids++
                            {{- end }}
                        }
                    {{- end }}
                    return res
                }

                {{if $object.Root}}
                    rrm := func(ctx context.Context) graphql.Marshaler {
                        return ec.OperationContext.RootResolverMiddleware(ctx, innerFunc)
                    }
                {{end}}

                out.Concurrently(i, func() graphql.Marshaler {
                    {{- if $object.Root -}}
                        return rrm(innerCtx)
                    {{- else -}}
                        return innerFunc(ctx)
                    {{end}}
                })

I would also like the name of the root resolver field so I am thinking about moving the WithFieldContext down into the field loop and add in the Field to the FieldContext. I have played around with adding a separate context as well but it seems like it fits within the FieldContext well.

	for i, field := range fields {
        {{- if $object.Root }}
            innerCtx = graphql.WithFieldContext(ctx, &graphql.FieldContext{
                Object: {{$object.Name|quote}},
                Field: field,
            })
        {{end}}

(Full diff of changes can be found here)

[CoreyWinkelmannPP]

Closing because PR #1647 was merged in.