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.

This is one of the most complicated (and frankly stupid) APIs I had to deal with... It almost seems like they designed each related endpoint to return slightly different data, and require different tokens/permissions/authentication type in order to get all the information that we need.

The following environment variables have to be configured for the dry-run workflow:

• ENTERPRISE_NAME: just rust-lang
• ENTERPRISE_APP_ID: app ID of sync-team-app-read (2885328)
• ENTERPRISE_APP_INSTALLATION_ID: installation ID of sync-team-app-read in the rust-lang enterprise (110701681)
• ENTERPRISE_APP_SECRET_KEY: secret key of the GH app (not gonna paste that in here :) )

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?

[marcoieni]

maybe if you update the branch the dry run will run? 🤔

[marcoieni]

But first, can you edit the github actions workflows to add the environments and secrets you mentioned? After, I will set them 👍

[Kobzol]

The dry run executed, but it failed, because the environment variables weren't set.

[marcoieni]

I think stuff like ENTERPRISE_NAME: rust-lang can be set from the workflow file directly, no? 🤔

[marcoieni]

I set the secret ENTERPRISE_APP_SECRET_KEY. 👍

[marcoieni]

Testing this PR in #2379

[marcoieni]

In #2379 I got the error

Cannot get organization installation for bors-rs of the enterprise app

You should filter orgs that are not part of the rust enterprise.

[marcoieni]

or maybe #2381 is the fix for this 🤔