Add support for client that uses GET as transport mechanism (#186)

Current implementation always uses POST as the transport mechanism. Adding GET support enables usage of GET queries for caching simply via URL.

Some notes:
- I left the existing API for creating a new client as is, but the implementation could be much cleaner by introducing some sort of configuration struct when creating a new client
- The construction of the query parameters follows the logic from Apollo's client implementation, which can be found here https://github.com/apollographql/apollo-client/blob/8beb4820edc6352996e08f7f73bde3573f1eb666/src/link/http/rewriteURIForGET.ts
- Updated integration tests to use both sets of clients. Updating the tests to use a test suite would be cleaner

internal/integration/server/gqlgen_models.go

@@ -34,6 +34,10 @@ type Hair struct {
 	Color *string `json:"color"`
 }
 
+type NewUser struct {
+	Name string `json:"name"`
+}
+
 type User struct {
 	ID          string  `json:"id"`
 	Name        string  `json:"name"`