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-CustomMarshalSlice.graphql-CustomMarshalSlice.graphql.go

@@ -208,25 +208,30 @@ func CustomMarshalSlice(
 	datesss [][][]time.Time,
 	datesssp [][][]*time.Time,
 ) (*CustomMarshalSliceResponse, error) {
-	__input := __CustomMarshalSliceInput{
-		Datesss:  datesss,
-		Datesssp: datesssp,
-	}
-	var err error
-
-	var retval CustomMarshalSliceResponse
-	err = client.MakeRequest(
-		nil,
-		"CustomMarshalSlice",
-		`
+	req := &graphql.Request{
+		OpName: "CustomMarshalSlice",
+		Query: `
 query CustomMarshalSlice ($datesss: [[[Date!]!]!]!, $datesssp: [[[Date!]!]!]!) {
 	acceptsListOfListOfListsOfDates(datesss: $datesss)
 	withPointer: acceptsListOfListOfListsOfDates(datesss: $datesssp)
 }
 `,
-		&retval,
-		&__input,
+		Variables: &__CustomMarshalSliceInput{
+			Datesss:  datesss,
+			Datesssp: datesssp,
+		},
+	}
+	var err error
+
+	var data CustomMarshalSliceResponse
+	resp := &graphql.Response{Data: &data}
+
+	err = client.MakeRequest(
+		nil,
+		req,
+		resp,
 	)
-	return &retval, err
+
+	return &data, err
 }