Support associating mappers with clients, not just scopes

[lmsurpre]

Today, keycloak-config supports the addition of Mappers to client scopes that it creates.
We use that for adding Audience, Patient Id, and Group mappers on select scopes.

What might be nice is to support something similar but on the client level.
That way we don't need to attach the same mapper to all possible client scopes.