insights: Poll insight when data is backfilling

Created by: chwarwick

Adds a feature flag that allows the UI poll for updates when an insight is currently backfilling.

closes #35479 (closed)

Test plan

Verified with feature flag on/off that insights render properly on standalone and dashboard pages. Verified polling stops when backfill is complete Verified polling does not take place with just in time insights

App preview:

• Web
• Storybook

Check out the client app preview documentation to learn more.