Add Reviewers to merge request event webhook payload

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Proposal

Include project Reviewers in the Webhook trigger payload of a merge request event. We would like to alert/notify Reviewers of a project with any merge request activity.

i.e. the creation of a merge request, approval of merge request, merging of a merge request or closure of a merge request. Currently it seems only the author and assignee are included in the payload as documented here https://docs.gitlab.com/ee/user/project/integrations/webhook_events.html#merge-request-events

Related to #346116

PM: @dhershkovitch