Signed-off-by: Steve Coffman <steve@khanacademy.org>

Currently gqlgen sets Content-Type header to 'application/json'. There's
no easy way to change it or add additional headers.

This commit adds struct variable ResponseHeaders that can hold any
headers you want to be returned with response. It is standard
`map[string][]string` variable.

If user does not set this map, we default to the Content-Type
header with 'application/json' value - nothing will be changed
for existing users.

Usage:

as simple as:

```
headers := map[string][]string{
    "Content-Type": {"application/json; charset: utf8"},
    "Other-Header": {"dummy-post-header","another-value"},
}

h.AddTransport(transport.POST{ResponseHeaders: headers})
```

Added tests in transport/headers_test.go.

…error message (#2597)

Co-authored-by: kerry <kerry@oscer.ai>

* EntityResolver input type fix

fixes #2326 - when using `@entityResolver(multi: true)` the InputType of
the entity resolver may be in a different package (usually `model`).
The fix is to pull the types.Type of the resolver input, and use
templates.CurrentImports.LookupType in order to render it correctly
(possibly adding another import)

* entityResolver input type fix: update tests

change testdata/entityresolver/gqlgen.yml to use a dedicated package for
the model (as in the default sample yml), and run go generate.

before the input type fix, generation fails with errors like -
plugin/federation/testdata/entityresolver/generated/federation.go:338:17:
  undeclared name: MultiHelloByNamesInput
plugin/federation/testdata/entityresolver/generated/federation.go:354:21:
  undeclared name: MultiHelloMultipleRequiresByNamesInput
plugin/federation/testdata/entityresolver/generated/federation.go:362:17:
  undeclared name: MultiHelloMultipleRequiresByNamesInput

Signed-off-by: Steve Coffman <steve@khanacademy.org>

Signed-off-by: Steve Coffman <steve@khanacademy.org>