Fix FeatureFlagClient to handle race conditions calls

Created by: erzhtor

Part 2 of https://github.com/sourcegraph/sourcegraph/issues/35543. Follow-up https://github.com/sourcegraph/sourcegraph/pull/35531.

Description

There are cases when initial calls to the FeatureFlagClient instance happen before the fetch request (which is called in the constructor) completes. This PR:

• Fixes it to always return fetched results
• Also, updates useFeatureFlag to use useObservableWithStatus
• Updates withFeatureFlag to allow rendering two components (when flag=true and when flag=false) and adds unit tests to

Test plan

• Unit tests added

App preview:

• Web
• Storybook

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