Added the featureFlagEnabled query field

What does this MR do and why?

The MR introduces a new feature flag resolver in the GraphQL API. This resolver allows users to check if a specific feature flag is enabled for the current user. The feature flag resolver is accessible through the featureFlagEnabled field in the GraphQL schema.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

No visual change

How to set up and validate locally

1. Open the GraphQL Explorer at http://127.0.0.1:3000/-/graphql-explorer (if your local GDK is running on http://127.0.0.1:3000/)
2. Add a query variable, and the query as presented on the screenshot (pick any existing flag you wish instead of ai_duo_chat_switch if you wish) and run the query

Referenced Support instance feature flags in VSCode (gitlab-vscode-extension#1349 - closed)