Log individual user usage of Slack service [RUN ALL RSPEC] [RUN AS-IF-FOSS]

What does this MR do?

It logs individual user usage of the slack integration. I added an event for every event that the slack integration can notify about. The only exception are pipeline events because those don't include a user ID.

This MR is pretty similar to !52110 (merged)

Related issues

• #297402 (closed)
• Feature flag rollout issue

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry (Not needed because this is not a user facing change)
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability 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
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done