saml: Rework SP-initiated logout config to support IdP-level config.

This gives more flexibility on a server with multiple organizations and
SAML IdPs. Such a server can have some organizations handled by IdPs
with SLO set up, and some without it set up. In such a scenario, having
a generic True/False server-wide setting is insufficient and instead
being able to specify the IdPs/orgs for SLO is needed.

zproject/settings_types.py

@@ -12,6 +12,7 @@ class SAMLIdPConfigDict(TypedDict, total=False):
     entity_id: str
     url: str
     slo_url: str
+    sp_initiated_logout_enabled: bool
     attr_user_permanent_id: str
     attr_first_name: str
     attr_last_name: str