Add support for Jira transition webhooks

Jira has two kinds of webhooks:

1. Event webhooks that are triggered when a Jira event happens, for example an issue is updated. This is supported by the Aha! Jira integration

2. Transition webhooks that are triggered when a Jira issue changes status. This is not currently supported by the Aha! Jira integration

More information about transition webhooks can be found at https://developer.atlassian.com/server/jira/platform/webhooks/#adding-a-webhook-as-a-post-function-to-a-workflow

Idea 1: document that transition webhooks are not currently supported

Idea 2: implement support for transition webhooks

Reason: event webhooks have performance problems in large Jira instances because they run a JQL query for every event. We have hundreds of events/second so Jira performance suffers. We can only use transition webhooks to integrate Jira with other systems.

Workaround

Since the payloads for event and transition webhooks differ only by a few values it is possible to build a bridge server that converts a transition payload to an event payload. This is extra work that could be avoided if the Aha! integration also accepted the transition payload.