feat: add as_maybe_shared() to HeaderValue

[martintmk]

Expose the inner Bytes via &dyn Any so callers can downcast and clone the shared handle without copying the underlying data.

This is alternative to #811 and #817 that does not expose any new external APIs while still giving performance-aware adopters the access the underlying Bytes instance.

let val = HeaderValue::from_maybe_shared(bytes::Bytes::from_static(b"hello")).unwrap();

let recovered: Bytes = val
     .as_maybe_shared()
     .downcast_ref::<bytes::Bytes>()
     .unwrap()
     .clone();