Add the ability to add sequential approvals for merge requests

This feature proposal was created on behalf of a GitLab Ultimate customer (internal link) who requested this functionality in a support ticket (internal link)

Proposal

Currently, if there are multiple approval rules on a merge request, they operate in parallel, and there is no sequencing in approvals.

It would be beneficial to configure sequential approvals, where a second approval rule is prompted for approval only after the first approval rule has been approved.

Example:

Approval Rule 1    0 of 3 approvals
Approval Rule 2    Pending Approval Rule 1