Add P2C load balancing scheme to App Proxy

P2C (power of two choices) is a standard-ish load balancing mechanism used in Traefik by default.

We chose WRR (weighted round robin) to allow dynamic reconfiguration of replicas, but for the purpose of stable services, it may be better to use a more “stable” load balancing scheme that has less skews time to time.

refs)

- [https://go-zero.dev/en/docs/tutorials/service/governance/lb#p2cpower-of-two-choices](https://go-zero.dev/en/docs/tutorials/service/governance/lb#p2cpower-of-two-choices)

This work will be a revisit of [PR #3221.](https://github.com/lablup/backend.ai/pull/3221)

JIRA Issue: BA-1989