feat: tower should be an optional dependency

[sigoden]

The tower/tower-service is only used in the client/client-legacy feature. If one only needs server/server-auto feature, there is no need to include them.

Testing in sigoden/dufs#321, with this pr, there is a reduction of 0.3 M in the stripped release binary.

[tottoto]

Sounds reasonable to me.

[seanmonstar]

Me too, thanks!

[seanmonstar]

Ah, looks like there's some cfg adjustments needed.

[davidpdrsn]

Is this a breaking change?

[tottoto]

As TowerToHyperService has not been released, I do not think this is a breaking change.

[sigoden]

The service features cannot be used independently.

hyper-util/src/service.rs

Line 30 in 99409f5

impl<S, R> hyper::service::Service<R> for TowerToHyperService<S>

#[cfg(all(
    any(feature = "http1", feature = "http2"),
    any(feature = "server", feature = "client")
))]
pub use self::service::Service;

https://github.com/hyperium/hyper/blob/d9c5d3b9b3921e05040ef50d4aca080f563aac36/src/service/mod.rs#L30-L33

[sigoden]

As TowerToHyperService has not been released, I do not think this is a breaking change.

As @tottoto said, this is not a break change. @davidpdrsn