Add ability to re-exec into shell when using tsh proxy kube.

[tigrato]

This PR introduces the tsh proxy kube --exec mode which spawns the kube proxy in the background and re-execs the user shell with the configured kubeconfig file.

Changelog: Added tsh proxy kube --exec mode that spawns kube proxy in the background, re-executes user shell with the appropriate kubeconfig.

[taraspos]

When using --exec following prompt still shows up:

Use the following config for your Kubernetes applications. For example:
export KUBECONFIG=/Users/username/.tsh/keys/...
kubectl version

Also, file mentioned in export KUBECONFIG doesn't exist, and the one in /var/folders/ used instead.

$ echo $KUBECONFIG
/var/folders/w3/njlgtc9j40z_0fkd3hrcz3bc0000gn/T/proxy-kubeconfig-113011939

So I would imagine we would want to remove the prompt if --exec mode is used.

[tigrato]

When using --exec following prompt still shows up:

Use the following config for your Kubernetes applications. For example:
export KUBECONFIG=/Users/username/.tsh/keys/...
kubectl version

Also, file mentioned in export KUBECONFIG doesn't exist, and the one in /var/folders/ used instead.

$ echo $KUBECONFIG
/var/folders/w3/njlgtc9j40z_0fkd3hrcz3bc0000gn/T/proxy-kubeconfig-113011939

So I would imagine we would want to remove the prompt if --exec mode is used.

I fixed the message. It was using the incorrect message. Now it prints:

Started local proxy for Kubernetes Access in the background.

Warning! Teleport will initiate a new shell configured with kubectl for local proxy access. 
To conclude the session, simply use the "exit" command. Upon exiting, your original shell will be restored, 
the local proxy will be closed, and future access through this headless session won't be possible.

 To work with different contexts use "kubectl --context", for example:
"kubectl --context='staging' get pods".
"kubectl --context='dev' get pods".

[public-teleport-github-review-bot]

@tigrato See the table below for backport results.

Branch	Result
branch/v13	Failed
branch/v14	Create PR
branch/v15	Create PR