Enhancement: Enforce OIDC auth flow via branding

A new branded parameter `enforce_oidc` has been added to enforce the app to follow the OIDC auth flow,
and `clientId` and `clientSecret` are sent in token requests when required by server. Moreover, the
app now supports branded redirect URIs with path due to the new branded parameter
`oauth2_redirect_uri_path` (legacy `oauth2_redirect_uri_path` is now `oauth2_redirect_uri_host`).

https://github.com/owncloud/android/issues/4500
https://github.com/owncloud/android/pull/4516
