Return User List Strategy to Unleash Clients

What does this MR do?

Support returning a strategy using a user list to Unleash clients.

• Return a userWithId strategy with the list of user ids found in the UserList to Unleash clients.
• Add a strategy type, gitlabUserList.
• Add validations for attaching a user list to a strategy.
• Refactor feature flag strategy spec to use let_it_be(:project) to improve performance.

Issue: #35930 (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