Have more fine grained actions on merge event webhooks

Problem to solve

Currently the action field for merge events is quite limited with a large number of individual actions grouped under "update". This makes it hard to differentiate between the multitude of changes that an MR can go through during its lifecycle, as editing the title, adding a label and pushing new commits all look the same from the webhook

Intended users

• Sasha (Software Developer)
• Devon (DevOps Engineer)

Proposal

It would be great if we could have different actions for distinct and separate events such as,

• assigned / unassigned
• labelled / unlabelled
• edited (description / title etc)
• pushed (for changes to git state)

This would allow for receivers to make more intelligent decision on how to react to the event.

Documentation

Webhook docs page would have to updated. It currently doesn't document the different action types (although fixing that is probably a separate issue)

What does success look like, and how can we measure that?

The action field of the MR event would have a more varied set of values

{
  "object_attributes": {
    "state": "opened",
    "action": "labelled"
  }
}