Implement tracking number of unique users pushing MRs for CI config [RUN ALL RSPEC] [RUN AS-IF-FOSS]

What does this MR do?

This MR adds a new usage event o_pipeline_authoring_unique_users_pushing_mr_ciconfigfile. This event will be triggered when a merge request, which contains the CI config file, is created or updated.

It is behind a feature flag usage_data_o_pipeline_authoring_unique_users_pushing_mr_ciconfigfile => #322166 (closed)

Related to #287682 (closed)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• 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

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