Track repository pushes as audit events

What does this MR do?

Tracks repository pushes to projects at the audit_events table behind the repository_push_audit_event feature flag.

This feature flag will remain disabled by default until we have a chance to tackle https://gitlab.com/gitlab-org/gitlab-ee/issues/7865, which is already scheduled for %12.4.

Note that this MR doesn't handle any major refactoring, but avoid adding more things to the AuditEventService (which is already doing a lot). Instead, it moves specific push audit logic to RepositoryPushAuditEventService.

Also important to note we're not tracking Wiki changes here.

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At this MR we create the structure to be able to insert into audit_events table in batches, asynchronously with a Sidekiq worker.

Issue: https://gitlab.com/gitlab-org/gitlab-ee/issues/1411

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created (https://gitlab.com/gitlab-org/gitlab-ee/issues/13986)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content