As we expand the Github driver, we're seeing a need to specify driver-specific
pipeline requirements. To accomplish this, bump the require/reject pipeline
keywords down a level underneath connection names. This lets users specify
per-source pipeline requirements.
This adds new API methods for sources to create the new pipeline filters
(by returning instances or subclasses of RefFilter, which used to be called
ChangeishFilter).
This change also creates and/or moves driver-specific subclasses of EventFilter
and TriggerEvent in(to) their respective drivers.
Change-Id: Ia56c254e3aa591a688103db5b04b3dddae7b2da4
aad3ae2fe1