Add GraphQL Extensions (#184)

This enables accessing the Extensions field, as defined in the response
format: https://spec.graphql.org/October2021/#sec-Response-Format

Extensions can be enabled using configuration option use_extensions.
This will change the return parameters of the generated client
functions. Making it a breaking change if enabled.

Since extensions are untyped as defined in the spec, the Client will
return an interface of type map[string]interface{}.

generate/testdata/snapshots/TestGenerate-Omitempty.graphql-Omitempty.graphql.go

@@ -227,20 +227,9 @@ func OmitEmptyQuery(
 	tz string,
 	tzNoOmitEmpty string,
 ) (*OmitEmptyQueryResponse, error) {
-	__input := __OmitEmptyQueryInput{
-		Query:         query,
-		Queries:       queries,
-		Dt:            dt,
-		Tz:            tz,
-		TzNoOmitEmpty: tzNoOmitEmpty,
-	}
-	var err error
-
-	var retval OmitEmptyQueryResponse
-	err = client.MakeRequest(
-		nil,
-		"OmitEmptyQuery",
-		`
+	req := &graphql.Request{
+		OpName: "OmitEmptyQuery",
+		Query: `
 query OmitEmptyQuery ($query: UserQueryInput, $queries: [UserQueryInput], $dt: DateTime, $tz: String, $tzNoOmitEmpty: String) {
 	user(query: $query) {
 		id
@@ -252,9 +241,25 @@ query OmitEmptyQuery ($query: UserQueryInput, $queries: [UserQueryInput], $dt: D
 	convert2: maybeConvert(dt: $dt, tz: $tzNoOmitEmpty)
 }
 `,
-		&retval,
-		&__input,
+		Variables: &__OmitEmptyQueryInput{
+			Query:         query,
+			Queries:       queries,
+			Dt:            dt,
+			Tz:            tz,
+			TzNoOmitEmpty: tzNoOmitEmpty,
+		},
+	}
+	var err error
+
+	var data OmitEmptyQueryResponse
+	resp := &graphql.Response{Data: &data}
+
+	err = client.MakeRequest(
+		nil,
+		req,
+		resp,
 	)
-	return &retval, err
+
+	return &data, err
 }