Expose Service and HttpService without features

[tottoto]

I would suppose that hyper-util would be good to allow users to enable http1 or/and http2 feature separately when using server-auto feature (although it is a breaking change). As hyper-util uses Service and HttpService to implement server-auto feature, these traits are required without http{1,2} features. Since these traits do not actually require optional dependencies, the drawback to expose them seems small comparing the benefit.

[tottoto]

This allows hyper-util to enable its service util without http1 and http2 features as well.

refs: hyperium/hyper-util#79

[tottoto]

hyperium/hyper-util#105 is a motivation for this proposal.