Allow different IPR checks for different branches

[dontcallmedom]

Some repos are used concurrently by a WG (for Rec-track work) and a CG (for incubation in the same area) - the repo manager should be able to distinguish which commitments are required for which pull requests, based on the target branch of the pull request, where a given branch would be for the WG, and another for CG work.

[tripu]

The new GH metadata file CODEOWNERS can associate specific maintainer(s) to specific branches, trees and/or files.

If that's flexible enough, it may make sense to reuse this file and its syntax (instead of coming up with a different mechanism) to decide what rules apply to each PR, depending on what branch and files it affects.