ci: Add workflow for tagging stable commit

[skoech]

Description

When a MicroCeph snap revision is promoted to stable, the corresponding Git commit is tagged manually.This PR adds a GitHub Actions workflow to help us automate updating Git tags (which we also use for marking our doc versions - and hopefully for release notes using the GitHub release feature). The workflow uses the github-script action, and will:

1. Install the MicroCeph snap,
2. Parse the snap info output,
3. Get the latest stable channel,
4. Find its commit ID,
5. Go to the repo and find that commit hash,
6. and tagging it.

The workflow will run on a regular cadence (twice a week).

Type of change

Delete options that are not relevant.

• Clean code (code refactor, test updates; does not introduce functional changes)

How has this been tested?

This workflow hasn't been tested. I am hoping to test it by triggering it manually first, before enabling the schedule event and cron expressions.

Contributor checklist

Please check that you have:

• [x] self-reviewed the code in this PR
• [x] added code comments, particularly in less straightforward areas
• [ ] checked and added or updated relevant documentation
• [ ] checked and added or updated relevant release notes
• [ ] added tests to verify effectiveness of this change