Add oidc support

[julien-nc]

This adds support for user_oidc in the GSS master node.
This requires nextcloud/user_oidc#1011

• Add a UserDiscoveryOIDC class to get the user location from an OIDC token attribute
• Update README
• Adjust MasterController::autoLogout to redirect to the user_oidc singleLogout URL if needed and pass the provider ID in the JWT logout token
• Adjust Master::handleLoginRequest to get the user information and location from user_oidc during login
• Adjust SlaveController::autoLogin to parse OIDC information in the GSS login JWT token
• Adjust Slave::handleLogoutRequest to insert the OIDC provider ID in the GSS logout JWT token