[FEAT] Add support for grouping external status checks

[kevinheins]

Is your feature request related to a problem? Please describe.
I have an external CI system (TeamCity) that reports status of commits back to a Github PR. These are considered statuses and not checks. The check-group app doesn't consider these statuses when collection checks.

Describe the solution you'd like
check-group can support external status inside sub-projects.

[tianhaoz95]

Thanks for pointing out this use case. From the doc https://docs.github.com/en/developers/webhooks-and-events/webhooks/webhook-events-and-payloads#status, it seem to contain all the information to perform the work.

Technically speaking, it won't be too hard to add this since we only need to add one more type of webhook to listen to and maybe slightly modify the parser.

2 considerations I have on top of my mind are: 1) how much traffic will it add for a repository that don't intentionally use statuses (maybe there are some GitHub statuses like GH pages), we might want to make it user configurable or filter the event somehow. 2) if it will be a bit confusing for users to define statuses the same way as check runs in the configuration file.

I will think a bit more about the 2 listed before adding a plan to support the use case.

If you have any good suggestions, please feel free to comment here :)