feat: pin snap revision, add metadata to control snaps

[ca-scribner]

Issue

(see #131 for a different approach to this problem)

The current charm does not pin the snap installed, instead installing it from latest/stable. This means that the snap version may float, potentially to a backwards incompatible version, without any intervention from the user.

The goals here are to:

• ensure the snap version installed is "held" (not upgraded on the fly during operation, without the admin asking for it). That way someone in the obs team pushing a new latest/stable cannot break existing deployments of the charm
• define what snap version any specific charm revision uses, making deployment predictable and reproducible (if someone deploys charm rev1 today and gets snap revA, they should get that same snap revA when they deploy again tomorrow, in a month, in a year, etc)

Solution

This PR pins the snap to a specific revision by adding:

• a snap manifest file (snap_installation.yaml) which defines the snap(s) to be installed by this charm and their channels or revisions
• tooling to install the snaps in the manifest, including setting snap refresh --hold=forever for any snap that is pinned to a revision
• tests for above

The snap manifest file is intended to make future maintenance easier, enabling CI to create PRs for future version bumps.

The snap management tools are modified from this

Testing Instructions

CI coverage should handle testing. If testing manually, you can juju ssh into the grafana-agent unit and check snap info grafana-agent, which should pin to revisionn 16 and say the snap has a "hold".

Upgrade Notes

Todo:

• [ ] add the revision for the amd snap

[ca-scribner]

I followed the solution from kubernetes since it did very similar things to what we wanted, but part of its complexity is because it handles different architectures, local vs store installs, etc. It ended up feeling a bit like overkill if implemented specifically for this charm, but it does work and if we put it in a package to share with other machine charms it could be really useful and worth the complexity

[ca-scribner]

Closed in favor of #131