Use NTP admin API instead of sled agent, remove sled agent time_sync API

[smklein]

Builds on #8555

This PR:

• Avoids the sled agent "zlogin to NTP zone" calls to chronyc, instead relying on the NTP admin server
• Removes the "timesync_get" API exposed from the sled agent, and moves all callers (RSS only) to call the NTP admin server instead

Fixes #8590

[smklein]

FYI, I ran this on a4x2 - it didn't work, until I fixed an SMF typo, now it works. This is tested implicitly by the "helios / deploy" job, which relies on RSS, and also relies on the timesync API in the NTP admin server.

[karencfv]

Looks great, thanks!

[karencfv]

I wonder if there's any documentation that relies on checking the response from this endpoint in the logs to debug an NTP sync issue or something similar. I have a vague memory of something like that. If anyone remembers, that documentation should be updated. Perhaps an announcement on Matrix might be good as well? I've certainly used it before if an omicron deployment was taking too long to start.