feat: Allow multiple values for SSO attribute_requirements via comma separation (#17949)

In the current `attribute_requirements` implementation it is only
possible to allow exact matching attribute values. Multiple allowed
values for one attribute are not possible as described in #13238.

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---------

Co-authored-by: Sebastian Neuser <pzkz@infra.run>
Co-authored-by: Quentin Gliech <quenting@element.io>

docs/usage/configuration/config_documentation.md

@@ -3337,8 +3337,9 @@ This setting has the following sub-options:
    The default is 'uid'.
 * `attribute_requirements`: It is possible to configure Synapse to only allow logins if SAML attributes
     match particular values. The requirements can be listed under
-   `attribute_requirements` as shown in the example. All of the listed attributes must
-    match for the login to be permitted.
+    `attribute_requirements` as shown in the example. All of the listed attributes must
+    match for the login to be permitted. Values can be specified in a `one_of` list to allow
+    multiple values for an attribute.
 * `idp_entityid`: If the metadata XML contains multiple IdP entities then the `idp_entityid`
    option must be set to the entity to redirect users to.
    Most deployments only have a single IdP entity and so should omit this option.
@@ -3419,7 +3420,9 @@ saml2_config:
     - attribute: userGroup
       value: "staff"
     - attribute: department
-      value: "sales"
+      one_of:
+        - "sales"
+        - "admins"
 
   idp_entityid: 'https://our_idp/entityid'
 ```