Implement ClustersClientsMiddleware

[luizbafilho]

Implement the middleware responsible for fetching the cluster and creating clients for each and injecting them in the context.

That work should be based of #1556

Implementation Notes

• If no clusters configmap is present, we understand that as running in a non-multi-cluster setup, so just return that single cluster client in the clusters clients' lists

[Callisto13]

Recapping what was discussed in our planning meeting:

• Multi-cluster is not a feature of core
• So core cannot have any way to add multiple clusters
• So we will be doing the multi bit in Enterprise
  • Including all the config map hack bits
• From core's side we need to:
  • Define the clusters fetcher interface
  • Fulfil that interface by adding a single, in-memory cluster to a pool which is added to the context (basically doing what we do now, but putting the client somewhere else.

More tickets will be coming up for the rest 👍