Provide OAuth 2.0 client credentials flow so that OAuth 2 apps can also act "asApp"

The grant_type: client_credentials is a standard grant type for service-to-service auth in OAuth2.0 and auth0 supports it out of the box. It certainly would be a great feature to allow things like webhook registrations “asApp” for OAuth 2.0 apps. This would ensure feature parity with Forge apps in terms of authentication as well, so that OAuth apps can delegate dynamic webhook registration to a "remote service".