Add Linux-specific pidfd process extensions (take 2)
Continuation of #77168.
I addressed the following concerns from the original PR:
- make `CommandExt` and `ChildExt` sealed traits
- wrap file descriptors in `PidFd` struct representing ownership over the fd
- add `take_pidfd` to take the fd out of `Child`
- close fd when dropped
Tracking Issue: #82971