WIP: Add lookup methods to find applicable features

What does this MR do?

This is sort of a WIP/POC that scratches an itch of mine. As we enable more and more feature flags, knowing which apply to your current session/view might be handy (#202526) As an initial step, I wanted to explore how to even GET that data, and not finding anything in the existing code base, rolled my own.

This isn't performance tuned

There's a fair bit of existing memoization and performance-related comments in Feature so it probably is worth a closer look.

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