fix(oauth2): restore client_credentials in default grant types list

The gating happens via allSupportedGrants in server.go, not via the
allowed list. Without client_credentials in the defaults, the
intersection filter always excluded it even with the flag enabled.
This matches how the password grant works: present in defaults but
only activated when the corresponding config flag is set.

Signed-off-by: Mathias Gebbe <mathias.gebbe@gmail.com>

cmd/dex/serve.go

@@ -614,6 +614,7 @@ func applyConfigOverrides(options serveOptions, config *Config) {
 	if len(config.OAuth2.GrantTypes) == 0 {
 		config.OAuth2.GrantTypes = []string{
 			"authorization_code",
+			"client_credentials",
 			"implicit",
 			"password",
 			"refresh_token",