Implement github trigger requirement status

This allows using a connection specific requirement on the status of the
head of a PR. A list of statuses is accepted, just like the pipeline
requirement.

Update GithubRefFilter class to be more clear that it deals with
required statuses, whereas the event filter works with the status being
provided.

Change-Id: Ib91f6527bf1d8ff5fbc6434c8adaca1cd5e1ba6d
4 files changed