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/roundtrip.go

@@ -101,10 +101,25 @@ func (c *roundtripClient) MakeRequest(ctx context.Context, req *graphql.Request,
 	return nil
 }
 
+func newRoundtripClients(t *testing.T, endpoint string) []graphql.Client {
+	return []graphql.Client{newRoundtripClient(t, endpoint), newRoundtripGetClient(t, endpoint)}
+}
+
 func newRoundtripClient(t *testing.T, endpoint string) graphql.Client {
 	transport := &lastResponseTransport{wrapped: http.DefaultTransport}
+	httpClient := &http.Client{Transport: transport}
 	return &roundtripClient{
-		wrapped:   graphql.NewClient(endpoint, &http.Client{Transport: transport}),
+		wrapped:   graphql.NewClient(endpoint, httpClient),
+		transport: transport,
+		t:         t,
+	}
+}
+
+func newRoundtripGetClient(t *testing.T, endpoint string) graphql.Client {
+	transport := &lastResponseTransport{wrapped: http.DefaultTransport}
+	httpClient := &http.Client{Transport: transport}
+	return &roundtripClient{
+		wrapped:   graphql.NewClientUsingGet(endpoint, httpClient),
 		transport: transport,
 		t:         t,
 	}