Add proxy command to dashboard

[hanxiaop]

Please provide a description of this PR:
Add a new command to display Ztunnel admin dashboard UI.

[GregHanson]

I agree using istioctl dashboard envoy ds/ztunnel -n istio-system is kind of a poor way to go. And I do think the dashboard command should be made available for ztunnel. @hanxiaop would it make sense to have a single istioctl dashboard proxy command that works for either envoy or ztunnel instance?

Also, we may need to open an item for improving the ztunnel dashboard UI as well. Just comparing it with envoy's:

• ztunnel /config_dump does not return a pretty printed json object
• ztunnel /logging is unusable since it expects a POST and a level
• ztunnel currently doesn't have a /stats API on 15000, but it does have one on :15020/metrics
• /certs and /server_info is technically in the /config_dump, not sure if they need their own API endpoint

[hanxiaop]

I agree using istioctl dashboard envoy ds/ztunnel -n istio-system is kind of a poor way to go. And I do think the dashboard command should be made available for ztunnel. @hanxiaop would it make sense to have a single istioctl dashboard proxy command that works for either envoy or ztunnel instance?

Also, we may need to open an item for improving the ztunnel dashboard UI as well. Just comparing it with envoy's:

• ztunnel /config_dump does not return a pretty printed json object
• ztunnel /logging is unusable since it expects a POST and a level
• ztunnel currently doesn't have a /stats API on 15000, but it does have one on :15020/metrics
• /certs and /server_info is technically in the /config_dump, not sure if they need their own API endpoint

@GregHanson Sounds great. The last one seems redundant to me and we may include the API if there are really some needs. Just created an issue istio/ztunnel#638

[hanxiaop]

not stale

[hanxiaop]

@GregHanson PTAL, thanks

[GregHanson]

LGTM