oauth2/oidc sso enhancement

[limng06]

1. support discovery
   a easy way to config oauth2-endpoint
   https://www.rfc-editor.org/rfc/rfc8414.txt
2. oidc support of endsession-point
   https://openid.net/specs/openid-connect-rpinitiated-1_0.html#rfc.section.2
3. support of role-mappings
   config example such as: RoleMappings: ["RoleAdministrator|administrator"]
   This way, we don't need to manually assign role permissions anymore.