feat: add ability to unmarshal Upload from map[string]interface

[golanglemonade]

Summary:

When using the generated server client, the types come through as a map[string]interface{} rather than a graphql.Upload. This change allows the generic map to be unmarshalled into the Upload type:

example from gen_server.go, this currently fails because its coming through as a map[string]interface{} instead of a graphql.Upload

func (ec *executionContext) unmarshalNUpload2githubᚗcomᚋ99designsᚋgqlgenᚋgraphqlᚐUpload(ctx context.Context, v interface{}) (graphql.Upload, error) {
	res, err := graphql.UnmarshalUpload(v)
	return res, graphql.ErrorOnPath(ctx, err)
}

I have:

• Added tests covering the bug / feature (see testing)
• Updated any relevant documentation (see docs)

[golanglemonade]

this actually looks like a problem with the gqlgenc client, rather than the parser here, going to close this out. sorry for the noise. (ref incase anyone else runs into this: Yamashou/gqlgenc#237)