Add worker to process auto-merge after approvals are given

What does this MR do and why?

This MR is a follow-up of !121828 (merged) and adds a worker to process auto merge after approvals are given.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #407352 (closed) and #8128 (closed)