Introduce sidekiq worker for auto merge process

What does this MR do?

This is a follow-up of auto merge strategy abstraction, which will be used in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/13278.

Since AutoMergeProcessWorker triggered when a pipeline is finished - regardless of its status, there are no changes on the MWPS behavior. The pipeline status will be checked in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/28595/diffs#ea9ddf41c3d80119933c3eb56d91fb6758bebc66_0_40 anyway.

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