Avoid torturing the settings service with "ListRoles" request for
every incoming request to the proxy.
The role Mapping is refreshed if cached data is older than 5 minutes.
Add a UserRoleAssigner implementation that extract role names from the
users' claims and creates role assignments in the settings service based
on a configured mapping of claim values to ocis role names.
Closes: #5669
This moves the lookup and the creation of the users' role assignemt out
of the user backend into its own interface. This makes the user backend
a bit simpler and allows to provide different implemenation for the user
role assignment more easily.
