Check mergeability of MR asynchronously

What does this MR do?

Executing MergeRequests::MergeabilityCheckService#execute can be time consuming and can likely cause a request to timeout.

Moving its execution asynchronously via #async_execute will help in ensuring page and API requests that needs it can load faster.

A feature flag (async_merge_request_check_mergeability) has been added. The API doc for List merge requests and Get single MR endpoints are also updated.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• 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

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

#29984 (closed)