Add client_credentials to config overrides

applyConfigOverrides() introduced a default list of grant types that
were allowed which didn't include client credentials because we
introduced it from an external PR.

This change adds client_credentials to the list.

cmd/dex/serve.go

@@ -583,6 +583,7 @@ func applyConfigOverrides(options serveOptions, config *Config) {
 			"refresh_token",
 			"urn:ietf:params:oauth:grant-type:device_code",
 			"urn:ietf:params:oauth:grant-type:token-exchange",
+			"client_credentials",
 		}
 	}
 }