Re-add functionality to sync GitHub apps

[Kobzol]

This essentially reverts rust-lang/sync-team#128, plus adapts to code to changes that happened in the meantime.

It would be great to check in which repositories is RenovateBot currently installed, so that we can double check that the current team config represents that faithfully. Though a dry run should also tell us that.

Fixes: #1915

[github-actions]

Dry-run check results

[WARN  sync_team] sync-team is running in dry mode, no changes will be applied.
[INFO  sync_team] synchronizing crates-io
[INFO  sync_team] synchronizing github
[INFO  sync_team] 💻 Repo Diffs:
    📝 Editing repo 'rust-lang/bors':
      Rulesets:
          Ruleset for main
            No changes

[marcoieni]

To recap, we are still using a PAT to deploy, but we are using a GitHub App for the dry run.

I will give read access to the following resources to the sync-team-app github app:

[marcoieni]

From the dry run it looks like we are about to remove all apps:
#2257 (comment)

Maybe it's because the token doesn't have enough permission even after enabling those other fields?