add support for codebuild github PR refs pattern

[mufaddal-rohawala]

Im from SageMaker Python SDK Team, we had to recently switch the codeebuild PR triggers to
SourceVersion = refs/pull/{pr-number}/head^{full-commit-SHA}) pattern.

To continue to support PR Comments via this fantastic tool, I wish to add support for this pattern.

More Info in: https://docs.aws.amazon.com/codebuild/latest/userguide/sample-source-version.html

Let me know if more details are needed.

[jlhood]

Apologies for the delay on this. The change looks like it's not going to break existing usage. I haven't personally tested the new regex pattern though. Did you? I'm happy to merge this. I'll verify it doesn't break existing functionality, but I'll need your help verifying the new support for refs. Will ping you offline re: testing.

Note, I checked the CodeBuild failure and it's unrelated to your change so I'm going ahead and merging even with the CodeBuild failure. I'll fix that separately.