Multiple hub architecture for a managed cluster

[apphe]

Allow a cluster to be managed by multiple hub clusters:

We may have use cases where different entities will want to leverage Open Cluster Management technology on the same clusters. For example below: a Red Hat SRE team may want to adopt multicluster engine to help manage their fleet of ROSA, OSD, ARO, ROKS clusters. The customer of those clusters may also want to purchase RHACM and import those clusters to use App, GRC, Observability, etc features.

[elgnay]

It's possible to support multiple hub architecture with hosted mode. That means run at most one Klusterlet on the managed cluster and the others outside of the cluster in the hosted mode.

With this solution, a Klusterlet (no matter it is installed in default mode or hosted mode) is required for each hub. In future, we may consider to remove the hub affinity of the Klusterlet and share a Klusterlet with multiple hubs (#141)