Implement publish incident service

What does this MR do?

Part of #205168 (closed).

This MR implements yet another service which takes care of finding issues and passing them to specialized services to upload them to CDN. Adding this service makes the upcoming background job worker much easier and less involved.

This MR is behind a beta feature flag and we tracking the rollout in #210026 (closed).

This MR also contains some refactorings in individual commits.

📣 Please don't squash the commits 📣

Note, although a feature we don't provide a changelog entry as these changes are not user-facing. This service is used in the follow-up MR.

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