Code Insights: [BE] [FE] Create env feature flag for gql insight API

Created by: vovakulikov

Context

We have to introduce an environment non-overridable feature flag to be able to switch our setting-based with gql API. At the moment we use setting cascade to store features flat but they all could be easily overridden which doesn't fit in our tech constraints of data migration.

How to create env feature flag with jsContext object.

1. Extend FE jsContext type https://sourcegraph.com/github.com/sourcegraph/sourcegraph/-/blob/client/web/src/jscontext.ts
2. Extend GO struct with feature flag here https://sourcegraph.com/github.com/sourcegraph/sourcegraph/-/blob/cmd/frontend/internal/app/jscontext/jscontext.go?subtree=true
3. After that it should be possible to use window.context.<our feature flag> in any place within the app.
4. Replace this line with window.context.<feature flag> statement https://sourcegraph.com/github.com/sourcegraph/sourcegraph/-/blob/client/web/src/enterprise/insights/InsightsRouter.tsx?L57&subtree=true