Web: Move use feature flag to the shared package
Created by: vovakulikov
Background
In the https://github.com/sourcegraph/sourcegraph/pull/40772 we encountered a need to use useFeatureFlag outside of web package. Prior to this PR this wans't possible because all logic around useFeatureFlag was stored inside of the web package. This PR moves useFeatureFalg and the requestGQL method inside of the shared package in order to import them in different packages (search, search-ui, web, ...)
Test plan
- Make sure that CI is green
App preview:
Check out the client app preview documentation to learn more.