This PR implements OpenID Connect Back-Channel Logout.

• New AS extension BackchannelLogoutExtension
• New AuthorizationServer.get_extension method to access it with server.get_extension(BackchannelLogoutSender)
• BackchannelLogoutExtension.send_logout sends the logout JWT to all the matching clients.

fixes #560

Checklist

• The commits follow the conventional commits specification.
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• You wrote unit test to demonstrate the bug you are fixing, or to stress the feature you are bringing.
• You reached 100% of code coverage on the code you edited, without abusive use of pragma: no cover
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