policy manager: Malcolm policy subscription and application

[mmguero]

@mmguero cloned issue idaholab/Malcolm#574 on 2024-10-01:

sub-item of this meta-issue: "policy manager" for Malcolm and Hedgehog Linux (idaholab#477)

This issue involves how Malcolm itself gets the contents of policies stored on itself.

So while Malcolm is managing its Git repositories (see idaholab#572), I can envision that we want to have maybe one set of policies for uploaded data and another set of policies for live captured data. I don't want to get as complicated as a different policy per uploaded PCAP file.

I think also you should be able to subscribe to multiple branches, for example, a "global" branch as well as a "local" branch. These would need to be prioritized and then flattened out (which could be done using rsync or something like that).