✨(backend) add option to configure list of required OIDC claims

We want to be able to refuse connection for users who have missing
claims from a list of required keys.

src/backend/impress/settings.py

@@ -474,6 +474,9 @@ class Base(Configuration):
         environ_prefix=None,
     )
 
+    USER_OIDC_REQUIRED_CLAIMS = values.ListValue(
+        default=[], environ_name="USER_OIDC_REQUIRED_CLAIMS", environ_prefix=None
+    )
     USER_OIDC_FIELDS_TO_FULLNAME = values.ListValue(
         default=["first_name", "last_name"],
         environ_name="USER_OIDC_FIELDS_TO_FULLNAME",