OpenID Connect RP-Initiated Logout should be optional

[MarcialRosales]

Is your feature request related to a problem? Please describe.

Some customers have reported they get the error below when they try to authenticate on the management UI with OAuth 2.0 protocol:
OpenId Discovery endpoint https://someurl.com/.well-known/openid-configuration not compliant

This is because the Authorization Server does not support OpenID Connect RP-Initiated Logout and therefore it does not include the attribute end_session_endpoint in the payload returned by the OpenId Discovery endpoint.

Describe the solution you'd like

RabbitMQ management UI will only support OpenID Connect RP-Initiated Logout when the OpenId discovery endpoint includes the attribute end_session_endpoint.

If the attribute is not present, the user is only logged out from the management ui.

Describe alternatives you've considered

No response

Additional context

Documentation PR: rabbitmq/rabbitmq-website#1893.

[michaelklishin]

@MarcialRosales I have moved it here from rabbitmq-website because the change involves more than just a doc update.