We always throw if `id_token_hint` is missing when `post_logout_redirect_uri` is used

The spec RP-Initiated Logout https://openid.net/specs/openid-connect-rpinitiated-1_0.html says:

3. Redirection to RP After Logout
   In some cases, the RP will request that the End-User's User Agent to be redirected back to the RP after a logout has been performed. Post-logout redirection is only done when the logout is RP-initiated, in which case the redirection target is the post_logout_redirect_uri parameter value sent by the initiating RP. An id_token_hint carring an ID Token for the RP is also RECOMMENDED when requesting post-logout redirection; if it is not supplied with post_logout_redirect_uri, the OP MUST NOT perform post-logout redirection unless the OP has other means of confirming the legitimacy of the post-logout redirection target. The OP also MUST NOT perform post-logout redirection if the post_logout_redirect_uri value supplied does not exactly match one of the previously registered post_logout_redirect_uris values. The post-logout redirection is performed after the OP has finished notifying the RPs that logged in with the OP for that End-User that they are to log out the End-User.

Currently we always throw, that is, stop the execution when id_token_hint is missing when post_logout_redirect_uri is used: https://github.com/simplesamlphp/simplesamlphp-module-oidc/blob/4c254afa8a80eb869351bc6dc161479f1f4d54a2/src/Server/RequestRules/Rules/PostLogoutRedirectUriRule.php#L59

Reading the spec, since id_token_hint is RECOMMENDED, we should not stop the execution, but simply not redirect to the post_logout_redirect_uri. We could show our own "you are logged out" page instead.

• TODO mivanci check how conformance tests handle this situation

Start with src/Server/RequestRules/Rules/PostLogoutRedirectUriRule.php at the linked rule and read the RP-Initiated Logout specification section on post-logout redirection. Check how the conformance tests handle a missing id_token_hint, as noted in the TODO. Done means the request is not stopped solely for the missing hint and post-logout redirection is withheld unless the specification permits it.