Allow any strategies to be set in `enable`/`disable` Feature Flags API

What does this MR do?

This MR allows to set any Unleash strategies in the following endpoints

POST   /api/:version/projects/:id/feature_flags/:name/enable(.:format)                   - Enable a strategy of a feature flag for an environment
POST   /api/:version/projects/:id/feature_flags/:name/disable(.:format)                  - Disable a strategy of a feature flag for an environment

With this change, we can use a custom strategy GitLab in our dogfooding process.

Related: #14046 #9566 (closed)

Feature Flag

This allowance is behind the feature_flag_api feature flag. We will only enable the feature flag on our dogfooding project. For the other projects, we will not enable it until we've resolved #34664 (closed).

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

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