Trigger manually sync/full sync

[martinm82]

Prerequisites:

• Is the functionality available in the GitHub UI? If so, please provide a link to information about the feature.

• Is the functionality available through the GitHub API? If the functionality is available, please provide links to the
  API documentation (https://developer.github.com/v3/) as well as the Octokit documentation (https://octokit.github.io/).

• If the functionality is not yet available in the API, it would be helpful if you
  contacted support (https://support.github.com/) or posted in the Community Forum (https://github.community/). Please
  include a link to the forum post if you create one or a copy of the response from support.

New Feature

Imagine you have the situation where the safe-settings app did not configure a repository properly, because for whatever reason, e.g., webhook not send to the app, an application error, etc..

Wouldn't it be great if there would be at least the possibility to trigger manually a sync or full sync through a dedicated endpoint?

[decyjphr]

This was solved for lambda by exporting syncInstallation method in the index.js #313

Alternatively, we can create an HTTP endpoint in the app that would do the same.

[anderssonjohan]

@martinm82 I'm thinking I will try to set up a GitHub Actions workflow using cron + workflow dispatch (manual run) to call the lambda function.
I have just started using this app but find it hard to get feedback on what happened for the real run after PRs are merged to the main branch of the admin repo. It would be great to be able to re-run the changes on main and get feedback from the lambda back as a commit status. The dry run is enough for PRs but is not good enough for main IMHO.

[pydolan]

As noted on #378, I have a prototype runner that I'm using for running safe-settings via GHA. If we like this approach of calling syncInstallation via a separate script, I can open a PR (was just delaying since I forked off my changes in #588) update: PR opened in #604