Add the ability to add sequential approvals for merge requests
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This feature proposal was created on behalf of a ~"GitLab Ultimate" [customer](https://gitlab.lightning.force.com/lightning/r/Account/0018X00003Hs79iQAB/view) (internal link) who requested this functionality in a [support ticket](https://gitlab.zendesk.com/agent/tickets/682595) (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
```
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issue