chore(ci): Import Firefox schema github workflow

[rpl]

This pull request defines a new github actions workflow which run the Firefox schema import and optionally creates an issue and a pull request.

The workflow can be called manually or being called from another workflow (so that we can define a workflow that runs the schema import and creates issue and pull request on a schedule, but also allow us to run it manually if we need to).

Fixed #5388

TODO list:

• [x] use github API to detect changes applied to the target import branch
• [x] Look into the circle-ci failure (it is actually unrelated to the changes in this PR, it is due to recent changes on the web-ext side, which requires nodejs >= 18, separate PR #5418 is meant to deal with it)
• [ ] create a workflow to run beta schema import automatically on a weekly basis
• [ ] create a workflow to test for breakage central schema import automatically on a weekly basis (and file issue if import or tests are broken)