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.
Jörn Friedrich Dreyer