Prepare for content-type negotiation

Combine the fields that will be used for content transformation
(content-type, codec, and group version) into a single struct in client,
and then pass that struct into the rest client and request. Set the
content-type when sending requests to the server, and accept the content
type as primary.

Will form the foundation for content-negotiation via the client.

pkg/kubectl/rolling_updater_test.go

@@ -1328,7 +1328,7 @@ func TestUpdateWithRetries(t *testing.T) {
 			}
 		}),
 	}
-	clientConfig := &client.Config{GroupVersion: testapi.Default.GroupVersion()}
+	clientConfig := &client.Config{ContentConfig: client.ContentConfig{GroupVersion: testapi.Default.GroupVersion()}}
 	client := client.NewOrDie(clientConfig)
 	client.Client = fakeClient.Client
 
@@ -1425,7 +1425,7 @@ func TestAddDeploymentHash(t *testing.T) {
 			}
 		}),
 	}
-	clientConfig := &client.Config{GroupVersion: testapi.Default.GroupVersion()}
+	clientConfig := &client.Config{ContentConfig: client.ContentConfig{GroupVersion: testapi.Default.GroupVersion()}}
 	client := client.NewOrDie(clientConfig)
 	client.Client = fakeClient.Client