feat: implement container_pid_to_host_pid()

[kyujin-cho]

This PR implements ai.backend.agent.utils.container_pid_to_host_pid() function, which has been left as TODO.

[sanxiyn]

This PR needs an update for cgroup v2 support(#865).

[achimnol]

It's a good refactoring.

Though, get_conatiner_process_table() could be reimplemented using /proc/{pid}/status's NSPid field (ref: https://man7.org/linux/man-pages/man5/proc.5.html) following get_host_process_table(), as long as we're on the Linux kernel 4.1 or later, like the existing NSpid-based code path for converting individual pids. (A notable exception is CentOS 7... and we may keep the current implementation as a fallback for CentOS 7 users and macOS development setups.)

It would be better to avoid spawning subprocesses for this kind of simple queries whenever possible. Even the mapping query itself could be rewritten to just read a single line of /proc/{pid}/status without contacting the Docker daemon API at all. I think reading the whole container process table could be also using the procfs NSpid field.

I think we could rewrite get_container_process_table() as a follow-up PR.