Resolve "Repeated calls to Redis for Flipper feature flag"

What does this MR do?

This memoizes Feature.get using RequestStore.

Are there points in the code the reviewer needs to double check?

I don't think so?

Why was this MR needed?

Because that could lead to a lot of Redis calls.

Does this MR meet the acceptance criteria?

• Tests added for this feature/bug
• Review
  • Has been reviewed by Backend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Closes #45528 (closed)