Notify events when auto merge is enabled or merge param is updated

What does this MR do?

Auto Merge facility receives bunch of event messages, for instance, when pipeline succeeds, it receives.

This MR registers a new event on when auto merge is enabled or merge param is updated i.e. (#execute). This way, the activated strategy can be processed right away if the condition is already fulfilled.

This is a preliminary work of https://gitlab.com/gitlab-org/gitlab-ee/issues/9186

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