Support OAuth2 Client Credentials grant type for App Connectors

Right now when configuring a Server as part of building an App Connector, only the Authorization Code OAuth2 grant type is supported.

It would be helpful to introduce support for the Client Credentials grant type ( https://auth0.com/docs/get-started/authentication-and-authorization-flow/client-credentials-flow/call-your-api-using-the-client-credentials-flow ) to support connections to APIs which do not support Auth Code grant types.