Extend MR icon in header with dropdown for assigned/review requests
## Problem to Solve
Users who have been assigned a merge request for review have no easy way to find the merge requests they need to review. This creates delays and friction in the review process for merge requests.
### Additional Details
Internally as GitLab has been using this feature - feedback was observed about the lack of discoverability of these requests:
> I’ve been using the new reviewers feature quite a bit lately and it is causing some confusion. I’ve had several MRs get delayed because the assigned reviewers had no idea they were assigned to the MR.
> anyone almost missing MRs because of the new reviewer field? I usually click on my assigned MRs list button at the top
> Has anyone else seen the Reviewer field? If I get assigned something here, it doesn't show up in my list of MRs.
## Proposal
As a follow-up to https://gitlab.com/gitlab-org/gitlab/-/issues/270417/, the first step in making MR review requests more accessible is by extending the MR icon in the header with a dropdown:

- The icon badge sums the counts of MRs where the current user is an assignee or reviewer.
- The dropdown has two options, each with the corresponding count of MRs: `Assigned to you` and `Review requests for you`.
- Each option links to the MR dashboard with the appropriate filter.
### Alternate Options
Alternative options were explored in [this comment thread](https://gitlab.com/gitlab-org/gitlab/-/issues/270417#note_449162985) with an ultimate decision that while a sectioned list approach of some kind might make the most long term sense, it's not something that could easily be implemented inside of a single iteration to solve the **immediate** pain users are experiencing.
We also recognize that introducing an additional click in this process is not an ideal solution, but a trade-off that we needed to make today.
issue