[Design] Reviewer assignment drawer post-feedback
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### Proposal
We can link the drawer from the "Assign" button in the sidebar as well as other areas of the merge request like in the "All required approvals must be given" merge check.

:tv: [Walkthrough](https://youtu.be/tiIztavTt7A)
:joystick: [Prototype](https://www.figma.com/proto/4KVQUkUDoZilkO4D1o1bvw/branch/H2fXj0CovyDCeBHXnLOqTY/Merge-request?page-id=11176%3A103188&node-id=11392-41494&node-type=CANVAS&viewport=-2090%2C-4165%2C0.11&t=gOhUxdq4jwSDk1Kp-1&scaling=min-zoom&content-scaling=fixed&starting-point-node-id=11392%3A41494)
#### Isn't this just quick assign
Quick assign was the concept of putting a `+` next to `Edit`. I personally never liked it because it didn't relate to why I would want to do quick add versus the drawer? Why would someone choose slow over quick?
The small change is linking the assignment drawer to required approvals.
> You have required approvals that you need to assign reviewers to
- `Requires X approvals`: Opens to show required rules
- `Approvals optional`: You don't need to assign reviewers but the rules exist. There might be workflows where teams use optional approval rules because there isn't enough logic to trigger them into required ones.
- Approvals are not setup: User may be using a free instance or have not setup approval rules or CODEOWNERS so in this case scenario, the experience is the same as the current.
In fact the "Edit" experience of the dropdown is all the same for the different cases of approvals being required/optional/disabled.

#### Foundation
The reviewer assignment drawer can be the foundation of automation features that make it easier to assign reviewers
<details><summary>Suggested reviewers</summary>
1. Concept 1: Suggested reviewers in dropdown
2. Concept 2: Suggested reviewers prefilled
- User needs to "assign" to lock in selection
3. Concept 3: Auto assign button
- Assigns required approval rules

</details>
<details><summary>Automated code review</summary>
Assigns "Duo Code Reviewer" as an additional reviewer

</details>
### Past concepts
- Separating the selection of reviewers and those that satisfy rules. [Video walkthrough](https://youtu.be/7VJT8ZbbWoA)
- Not going with this because `Approving` is an action and `Reviewer` is the role
- Inline assignment in the sidebar
- Not going ahead because of space concerns when there are many rules [inline-assignment video](/uploads/cb365372bc2a19f24395d1ec7f5e6627/inline-assignment.mp4)
issue