Expose known clusters from the clusters-manager

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What changed?

• Expose the known clusters in the clusters-manager API
• We already have subscribe, this allows you to query the existing clusters if you subscribe after some have been added.

Why was this change made?

• Allows us to get the config for clusters in case we need to make other kind of queries against them that the controller-runtime client does not support. For example ProxyGeting via the client-go.

How was this change implemented?

• Add a getter method to expose the internal clusters list.

How did you validate the change?

• Manually