Workflow to sync strings

[st3fan]

(Similar to mozilla-mobile/android-components#10075)

This patch introduces a sync-strings.yml GitHub Actions Workflow that will periodically copy strings from master to the Fenix Beta branch.

• It is configured to run daily at midnight UTC.
• It will only create Pull Requests, but not land them. I'd like to start with manually reviewing these string sync PRs for a while and if we feel they are good to auto-land we can introduce a Mergify rule for that.
• It will create PRs names automation/string-sync-$RELEASE and it will re-use the PR if new string changes need to be integrated and a previous PR has not been merged yet.

The GitHub Actions used in this workflow are either from Mozilla or GItHUb, except for the peter-evans/create-pull-request one. I've read through the code and I think it is trustworthy. (Good version number, well maintained, high number of users, officially on the GitHub Marketplace - but let's have a discussion if we have doubts)

The string sync logic is in https://github.com/mozilla-mobile/sync-strings-action

Pull Request checklist

• Tests: This PR includes thorough tests or an explanation of why it does not
• Screenshots: This PR includes screenshots or GIFs of the changes made or an explanation of why it does not
• Accessibility: The code in this PR follows accessibility best practices or does not include any user facing features. In addition, it includes a screenshot of a successful accessibility scan to ensure no new defects are added to the product.

To download an APK when reviewing a PR:

1. click on Show All Checks,
2. click Details next to "Taskcluster (pull_request)" after it appears and then finishes with a green checkmark,
3. click on the "Fenix - assemble" task, then click "Run Artifacts".
4. the APK links should be on the left side of the screen, named for each CPU architecture

[escapewindow]

Thanks! I have some questions, but I think they're blockers for autoland, not blockers for creating a PR.

[st3fan]

I changed the frequency to very 4 hours to make things a little easier for testing.