Make terminals for pods on workload clusters working on the control plane

[jannfis]

The Argo CD API server talks to a target Kubernetes API to stream the logs of a managed application's pod. This will not work in an agent based model. We need to figure out how we can support this use case with argocd-agent.

Similar to #128, some idea around this is having a Kubernetes API proxy for the Argo CD API server, which proxies pod log requests to the principal. If the appropriate agent is connected, it could be requested to stream the terminal connection to the principal, which in turn would send it via the Kubernetes API proxy to the Argo CD server. Not sure if that is feasible, though.

We need to think this through.