Turbopack: Add cache for reqwest clients (#81742)
We need a cache because:
- This will allow us to enable HTTP keep-alive in a subsequent PR which would allow connection-reuse.
- Using `rustls-tls-native` may make client construction far more expensive: #79060
Clients don't fit turbo-task's caching architecture and are not serializable, so the cache lives outside of turbo-tasks. I'm re-using the https://docs.rs/quick_cache/latest/quick_cache/ crate for this, which we already pull in for turbo-persistence.
