Relax `Sync` requirements on `Body` type

I believe we can remove Sync from Body's trait bounds. This follows the pattern that other libraries have taken such as tonic and axum which have Body types that do not require Sync.

Doing this allows warp::service to be more compatible across the Tower ecosystem. For example, if you really wanted to, you could do arcane things like:

// Build an Axum router which passes requests through warp filters
let axum_router = axum::Router::new().fallback_service(warp::service(warp_filters));
// Previously this would fail since Axum's `Body` type is not `Sync`