Restrict who can decline a pull request

Like we have Merge checks in Branch permissions, it would be useful if a similar functionality for who should be allowed to Decline a Pull Request.

The choices I would be looking for are:

• Author
• Reviewer

Looking at Decline specifically is important as it corresponds to a what-happens-next type of workflow.

For example:

• Is the issue still relevant?
• Should the ticket be updated with a different priority?
• Mark as `Won't Fix`, etc

This obviously depends on everyone's workflow, but for the above example it would the the Author's responsibility to do the above changes at the same time of Declining the Pull Request.

A similar suggestion can be found here:  BCLOUD-12509