[DEVEXP 391] Add generic webhook to trigger builds manually

@bparees @csrwng Please review
@smarterclayton Given your previous comment regarding build triggers, does it make sense to add versioning to the event objects?  Is it just going to change anyway?  Also, wouldnt the version be required as part of the payload so it is deserialized properly?  Do we version the GitHubPushEvent?  How would that structure get the proper version?