Add experimental.varyParams feature flag (#89307)
## Summary
- Adds `experimental.varyParams` feature flag (defaults to `false`)
- When disabled, the client skips vary params rekeying and treats params
as unknown
- Allows internal rollout with easy revert if regressions occur during
dogfooding
The flag only gates the client-side rekeying logic in `cache.ts`.
Server-side param tracking continues regardless.
## Test plan
- Enabled the flag in the vary-params and optimistic-routing test
fixtures
