prober: add a DERP probe manager based on derpprobe

This ensures that each DERP server is probed individually (TLS and STUN)
and also manages per-region mesh probing. Actual probing code has been
copied from cmd/derpprobe.

Signed-off-by: Anton Tolchanov <anton@tailscale.com>
This commit is contained in:
Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f