Docker Registry event handling (1/4)

What does this MR do?

We create an endpoint for Docker registry events here. This MR can be merged independently of the main feature as it's not enabled until Docker Registry is not configured specifically to send events to this endpoint. No feature flag needed.

CE backport https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/30520

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