MLPAB-2857 - create a new merge group workflow

[andrewpearce-digital]

Purpose

Run a new workflow in merge queues

Fixes MLPAB-2857

Approach

• create merge group workflow
• deploy to staging environment
• identify when running as a merge group trigger and create a pre-release tag

Learning

• https://docs.github.com/en/actions/writing-workflows/choosing-when-your-workflow-runs/events-that-trigger-workflows#merge_group
• https://docs.github.com/en/repositories/configuring-branches-and-merges-in-your-repository/configuring-pull-request-merges/managing-a-merge-queue
• https://docs.github.com/en/webhooks/webhook-events-and-payloads#merge_group