[POC] Remove hard-coded cluster list from multi-cluster POC

Follow on to CONSOLE-2748. The cluster list should no longer be hard-coded to one additional cluster. Get the list of clusters from ACM. The backend will need to be updated so that it can proxy to any cluster.

TODO:

• Determine if there is a way to get the CA file for the cluster if it's a self-signed certificate.
• Determine if the backend (or console operator) should request the cluster list or the frontend. Regardless there will be backend changes.

Non-goals:

• This story does not deal with version skew across clusters in the fleet. For the POC, we can assume all clusters will be at the same version.
• Assume the user is kubeadmin. We don't necessarily need to use the console service account to fetch the cluster list for this POC.

Technical Details:

Once you have ACM installed, the kube api server might be available at:

ManagedClusterInfo > spec.masterEndpoint
ClusterDeployment > status.apiURL (if cluster was provisioned from ACM/Hive)

For a quick list of managed clusters, you could query the projects by this label:

cluster.open-cluster-management.io/managedCluster

ACM has a namespace per managed cluster.

Acceptance Criteria:

• Working code that builds on the existing prototype and uses the ACM cluster list to populate the dropdown.
• Document any limitations or problems that arise during the prototype.