[Improvement] Add back k8s exec client in kb-agent

[cjc7373]

Lorry used to have a k8s exec client, which invokes lorry container through kubectl's exec capabilities. It is useful in local debug, as we often run kubeblocks as a process (with go run commands) instead of building an image and running as a pod.

In kb-agent, this exec client is missing and local debug becomes more complicated. I suggest add back exec client.

[cjc7373]

Using port forward to achieve the same functionality seems to be better, as it reuses the HTTP client. (So that we don't need to deal with things like escaping quotes)

[github-actions]

This issue has been marked as stale because it has been open for 30 days with no activity