feat(client): OAuth2 Client credential support (#259)

* Initial implementation

* Added OAuth2 support to `client` config

* Revert "Initial implementation"

This reverts commit 7f2f3a603ae018b1cd1c6a282104f44cd9a1a1d1.

* Restore vendored clientcredentials

* configureOAuth2 is now a func (including tests)

* README update

* Use the same OAuth2Config in all related tests

* Cleanup & comments

core/endpoint.go

@@ -111,7 +111,10 @@ func (endpoint *Endpoint) ValidateAndSetDefaults() error {
 	if endpoint.ClientConfig == nil {
 		endpoint.ClientConfig = client.GetDefaultConfig()
 	} else {
-		endpoint.ClientConfig.ValidateAndSetDefaults()
+		err := endpoint.ClientConfig.ValidateAndSetDefaults()
+		if err != nil {
+			return err
+		}
 	}
 	if endpoint.UIConfig == nil {
 		endpoint.UIConfig = ui.GetDefaultConfig()