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.
This removes the "withRoles" flag from the GetUserByClaims lookup and move the
functionality into a separate method. This should make the code a bit more readable
in preparation for maintaining the RoleAssignments from OIDC claims.
The old approach of the authentication middlewares had the problem that when an authenticator could not authenticate a request it would still send it to the next handler, in case that the next one can authenticate it. But if no authenticator could successfully authenticate the request, it would still be handled, which leads to unauthorized access.
