Prevent simultaneous requests on UserFinder - GitHub Import

What does this MR do and why?

Based on the #435340 (closed), we're preventing simultaneous requests on Gitlab::GitHubImport::UserFinder by introducing exclusive lease.

notes:

• we're scoping the lease key per project.id;
• the length of time the lock will be valid for is 3 minutes;
• the sleep time is 1 second;
• The maximum number of times we will re-attempt to acquire the lock is set to 30.

Screenshots or screen recordings

N/A

How to set up and validate locally

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 #435340 (closed)