Auto Merge - Add external status checks to auto merge capabilities

What does this MR do and why?

Add external status checks to the list of checks we perform for auto merging.

To test:

• Ensure merge_when_checks_pass and additional_merge_when_checks_ready feature flags are on
• Create an MR that is mergeable, while ignoring the external checks for now
• Go to settings/merge_requests of the project, and create a new status check http://localhost:8000
• Tick the setting that the external status check must pass
• The external status check should be 'pending' and we should be able to set to auto merge
• Set auto merge
• Send a curl request to pass the status check
• curl -X POST -d 'sha=SHA' -d 'external_status_check_id=ID' -d 'status=passed' -H "PRIVATE-TOKEN: TOKEN" "http://localhost:3000/api/v4/projects/PROJECT_ID/merge_requests/MERGE_REQUEST_IID/status_check_responses"
• Check that the MR is merged

Related to #440911 (closed)