[BB] Trigger pipelines only on raising/updating PRs

[harryi3t]

Usecase: I want to be able to trigger buildkite pipelines only when someone raises or updates a PR.

Problem: I am using Bitbucket and added a webhook to trigger my pipeline on buildkite. When I set the triggers Pull Request -> Created/Updated, it does not work. Only when I set the trigger Repository -> Push it works, but the problem with this is that the pipeline will be triggered as soon as someone created a new branch with some commits but haven't raised a PR yet. Doesn't look like a Bitbucket issue since they are sending the webhook correctly (It is shown in the webhook history and received 200 response from buildkite)