Return result of onLogout

The auth-saml plugin would benefit from supporting a full logout process.

Use case:

• The user logout from peertube
• The auth-saml plugin craft a logoutUrl that the user must visit to complete its logout process
• The server respond to the client with that logoutUrl as redirectUrl
• The client redirect the user to the redirectUrl

This MR contains the following changes:

• Give access to the request during the onLogout call
• Use the return value of the onLogout method of an auth provider as the redirectUrl
• Forward the logoutUrl to the client in the revoke-token response
• The client redirect the user to the logoutUrl if it is exists