Add system/comment filter to notes api

What does this MR do?

This adds a filter to the notes API in order to filter to "system" vs "non-system", or as the UI calls it "comments" vs "activity". In keeping with the same symbolism as the UI uses, I have actually used the same symbols as the API filter values.

This is to alleviate the pain in #25968 (closed). I don't have the knowledge to better improve that situation, but I can reduce the pain for our specific workload by allowing us to filter out the system comments, which don't interest us.

Screenshots

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

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