make AccessRequest return a unique prefix

[Diaphteiros]

What would you like to be added:
Currently, the AccessRequest is expected to return just a secret reference with access to the Cluster.

Originally we had planned that AccessRequests additionally return a unique prefix (at least for shared clusters) that the controller which created the AccessRequest can use to prefix cluster-scoped resources. This way, we can ensure that there are no naming conflicts between multiple controllers accessing the same shared cluster.

Do we still need this feature?

Why is this needed:
Avoid naming conflicts for cluster-scoped resources in shared clusters.

[Diaphteiros]

Some thoughts:

• So far, everything worked fine without this feature. But the only cluster where this is relevant for now is the onboarding cluster, and there our user management ensures separated namespaces. This becomes more interesting for the workload clusters.
• The generated prefix must be stable. The best idea would be to generate it from the AccessRequest's metadata in a deterministic manner.