BitBucket Server Importer - Preserve PR (MR) reviewers

What does this MR do and why?

As part of the #416611 (closed), we are adding reviewers to merge requests.

For finding a reviewer, we're using UserFinder class with .find_user_id(by: :username, value: reviewer['username']) method.

For saving/importing reviewers, we're using MergeRequestReviewer.insert_all to avoid N+1 INSERT INTO queries.

Screenshots or screen recordings

Screen_Recording_2023-07-24_at_15.08.20

How to set up and validate locally

1. Set up the Bitbucket Server locally by following the docs.
2. Create a project on BitBucket Server, open an MR and assign a reviewer.
3. Use the Import projects from Bitbucket Server feature and import the project from step 2.
4. Validate that newly imported project has MR imported with the correct reviewer(s).

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #416611 (closed)