Add managed process crate

[MOZGIII]

This PR is a part of the work towards the workers refactor; it introduces a crate for managed processes which is to be used in the worker pools.

This is a concise building block that encapsulates the behaviors of a process that needs to behave in a suitable-fro-the-process-pool manner: it has to terminate when we exit, even if we got SIGKILL-ed and are unable to gracefully terminate the process pool.

This crate is not used just yet, but the plan is to build new set of concise crates on top of it, and eventually replace the waymark-remote crate. For a preview of the full picture check out this PR: #278.

[github-actions]

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