API: Add labels to note event payload

Hi there,

Description

It would be useful to get the list of labels existing on an issue when the note event is triggered but the issue payload doesn't currently include a label field. It is loosely related to #33062 (moved) as note events could be used to inspect current label status instead of a dedicated label events.

Proposal

Add a label field to the issue object in the "Comment on Issue" comment event.

Links / references

https://docs.gitlab.com/ce/user/project/integrations/webhooks.html#comment-on-issue

Documentation blurb

  "issue": {
    ...
    "labels": "this,that,another",
    ...
  }