Add an automatic node self-update feature

[ljedrz]

This PR aims to replace the existing auto-update scripts by performing periodic checks on a specified branch of snarkOS, downloading any updates, rebuilding the node and restarting it with the same arguments as the ones it was started with. It also uses git and expects the host to be able to build snarkOS.

It is feature-gated and currently not enabled by default (but it probably should be once we decide to use it and update the docs) and - not unlike the scripts - only works on Unix-like systems. Btw, we might want to recommend that users use WSL by default due to our clang build requirement regardless.

Open points:

• [ ] when should it become a default feature
• [ ] what to do with the existing scripts
• [ ] should the auto-update check interval be changed (I set it to 15 minutes)

[ljedrz]

the clippy job failure is unrelated: https://github.com/briansmith/ring/issues/1469

[ljedrz]

Rebased.

[ljedrz]

Rebased.

[aleo-svc-exp]

Admin: Please respond with "build this" in order to build this PR.