feat(apply): magic rollback

[water-sucks]

Description

In case changes in configuration settings for connections (i.e. SSH, internet interfaces, etc.) restart and cause loss of access to the machine in question, this PR implements a magic rollback mechanism that runs on the target.

It creates a new binary, activation-supervisor, that lives in the /bin directory of the target's NixOS system closure, alongside a thin module that integrates supervisor binary into the system closure.

Closes #119.

[coderabbitai]

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