Add support for filtering on PR Titles for Bitbucket Cloud/Server

[jordan-simonovski]

Summary

I'd like to add the ability to match on pull request titles for Bitbucket Cloud/Server. This change would introduce a new matchFilter in the existing code, and create an additional way to look for pull requests to generate an appset for.

Motivation

Currently using the pull request generator which matches on branches, and continually running into problems where the branch name doesn't contain the particular string we're matching for. Being able to change the PR title on the fly so that Argo can pick up a change for a particular app to test would make this a game changer!

Proposal

I've already tested this out in my fork where the change is to essentially add a TitleMatch to the Filter struct as well as to the Pull Request Generator Filter in applicationset_types.go