Support New Feature Flags in Unleash API

What does this MR do?

Add support for new feature flags to the Unleash API (/feature_flags/unleash/:project_id/client/features).

This code is behind a feature flag as per the issue.

When the feature flag is enabled, return both legacy and new feature flags to Unleash clients.

When the feature flag is disabled, return only legacy flags to Unleash clients.

The new CRUD API for feature flags will be behind the same feature flag.

Issue: #204895 (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