Provide reviewers/participants in webhook data for pull requests

[shollander]

Feature Description

The json data posted by outgoing webhooks for pull request events currently contains assignees, but not reviewers or participants (both of which show in the UI as part of a PR).
It would be very useful to have a list of the reviewers and participants in the webhook data.

Screenshots

No response

[shollander]

I noticed that as of 1.20.0, requested_reviewers are included in some webhook events.
It seems they are included in review events, but not comment events. I would think they should be included in all PR events.

It would also be nice to have participants included in all events.