Add cluster management for Porch UI in Nephio context

[liamfallon]

Original issue URL: kptdev/kpt#3612
Original issue user: https://github.com/debeaueric
Original issue created at: 2022-10-11T20:27:07Z
Original issue last updated at: 2022-11-15T21:56:50Z
Original issue body: ### Describe your problem

In the Nephio context, we must be able to manage various clusters where the workloads (eg CNF) will be deployed
The current Porch UI does not manage clusters hosting the workloads.
As a result, the Synchronisation action ("CREATE SYNC") for a deployment only triggers the deployment on the
management cluster (hosting Porch UI).

The synchronisation action should list the potential clusters "managed" by Porch UI.
It also implies that Porch UI is able to manage a list of clusters.

Original issue comments:

[liamfallon]

Triaged
Triage Comment: Plans for the UI are unclear, but we should document bugs