feat: federated logout for SAML IdPs (#9931)

# Which Problems Are Solved

Currently if a user signs in using an IdP, once they sign out of
Zitadel, the corresponding IdP session is not terminated. This can be
the desired behavior. In some cases, e.g. when using a shared computer
it results in a potential security risk, since a follower user might be
able to sign in as the previous using the still open IdP session.

# How the Problems Are Solved

- Admins can enabled a federated logout option on SAML IdPs through the
Admin and Management APIs.
- During the termination of a login V1 session using OIDC end_session
endpoint, Zitadel will check if an IdP was used to authenticate that
session.
- In case there was a SAML IdP used with Federated Logout enabled, it
will intercept the logout process, store the information into the shared
cache and redirect to the federated logout endpoint in the V1 login.
- The V1 login federated logout endpoint checks every request on an
existing cache entry. On success it will create a SAML logout request
for the used IdP and either redirect or POST to the configured SLO
endpoint. The cache entry is updated with a `redirected` state.
- A SLO endpoint is added to the `/idp` handlers, which will handle the
SAML logout responses. At the moment it will check again for an existing
federated logout entry (with state `redirected`) in the cache. On
success, the user is redirected to the initially provided
`post_logout_redirect_uri` from the end_session request.

# Additional Changes

None

# Additional Context

- This PR merges the https://github.com/zitadel/zitadel/pull/9841 and
https://github.com/zitadel/zitadel/pull/9854 to main, additionally
updating the docs on Entra ID SAML.
- closes #9228 
- backport to 3.x

---------

Co-authored-by: Silvan <[email protected]>
Co-authored-by: Zach Hirschtritt <[email protected]>

console/src/assets/i18n/de.json

@@ -2341,6 +2341,8 @@
     "REMOVE_WARN_DESCRIPTION": "Sie sind dabei, einen Identitätsanbieter zu entfernen. Dadurch wird die Auswahl des verfügbaren IDP für Ihre Benutzer entfernt und bereits registrierte Benutzer können sich nicht erneut anmelden. Wollen Sie wirklich fortfahren?",
     "DELETE_SELECTION_TITLE": "Identitätsanbieter löschen",
     "DELETE_SELECTION_DESCRIPTION": "Sie sind im Begriff mehrere Identitätsanbieter zu löschen. Die dadurch hervorgerufenen Änderungen sind unwiderruflich. Wollen Sie dies wirklich tun?",
+    "FEDERATEDLOGOUTENABLED": "Federated Logout aktiviert",
+    "FEDERATEDLOGOUTENABLED_DESC": "Wenn aktiviert, wird der Benutzer auch vom IdP abgemeldet, wenn der Benutzer die Sitzung in ZITADEL beendet.",
     "EMPTY": "Kein IDP vorhanden",
     "OIDC": {
       "TITLE": "OIDC Konfiguration",