Add readiness/liveness probes to vizier-core

[lkpdn]

Currently we don't make much use of readiness/liveness probe, though there are major cases where if those are to be installed, it would help cluster admins or users recognize and diagnose what is going on when some component goes bad. Let me write some down as my first thought.

• 1). When vizier-db takes tens of seconds to boot up, which is quite normal especially if the components are cleanly deployed for the first time, vizier-core is unavailable in the meantime. We can tell vizier-core is ready once Katib manager successfully starts to listen.
• 2). Normal liveness probe against 0.0.0.0:6789 seems the best, though we cannot tell whether or not the backend db is healthy, which would heavily affect much of katib manager's gRPC methods.

[lkpdn]

Richer implementation has been merged. Close this one.