Consolidate auto-insertion pipelines

[RikkiGibson]

There's a significant amount of "sprawl" across our insertion pipelines--one for each release branch, one for PR validation, one for master validation, etc.

It would be great if we could consolidate all the pipelines that create draft insertions, for example. The insertion tool should be able to handle the variations between e.g. official builds for QB branches and validation builds by just looking at the tags on the build.

Ideally we could have a pipeline that creates draft/validation insertions and another pipeline that creates auto-merging insertions.

[RikkiGibson]

This is somewhat addressed by the Roslyn changes to perform insertions as part of the official build. Keeping open in case we decide to do something similar with OneOffInsertion/UpdateExistingPr/etc.