Allow different http client implementations

[jplatte]

To make them not mutually exclusive (for easier testing and less conditional compilation) we should use generics, i.e. have MatrixClient<Hyper<Rustls>> or MatrixClient<Isahc> or MatrixClient<Surf>. This is similar to what we did before with hyper::client::connect::Connect but easier since the types to plug in will be in the same crate (enabled by cargo features) rather than in separate crates.

We may also want to provide a DefaultMatrixClient / DefaultClient type alias with a different definition depending on which features are activated.