[New Auto Merge Strategy] Add To Merge Train When Pipeline Succeeds

What does this MR do?

This MR adds a new auto merge strategy - Add To Merge Train When Pipeline Succeeds.

This strategy basically does

• Wait until the latest pipeline in the merge request succeeded.
• Once it succeeded, the system adds the merge request to a merge train.

which is pretty close to MWPS feature. The only difference is adding the MR to a train or merge immediately.

Issue: https://gitlab.com/gitlab-org/gitlab-ee/issues/9186

This MR is on top of https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/13278 at this moment.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• [-] Separation of EE specific content

Performance 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