Draft: Move approval rules code for merge request creation into Sidekiq job [RUN ALL RSPEC] [RUN AS-IF-FOSS]

What does this MR do?

To reduce the number of SQL queries executed during the merge request creation API, move approval rules code into a sidekiq job.

The query to insert approval rules can also be slow. It sometimes takes over a second. It appears in the postgres slow log a couple of hundred times a week. See #20770 (comment 546947169).

The code has been moved into EE::MergeRequests::AfterCreateService, which is executed by NewMergeRequestWorker.

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because this is a performance improvement.
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Related to #20770 (closed)