Change `Connect` trait to alias `Service`

[seanmonstar]

After #1782 is done, the Connect trait should be changed to basically an alias for a Service<Destination>. This would allow more composability, being able to use ServiceExt to customize connectors.

[seanmonstar]

I just remember there is also MakeConnection in tower, perhaps that can just be used instead. One difference is that hyper's Connect currently returns a Connected metadata type.

[LucioFranco]

@seanmonstar I totally missed this issue! But we should drop the connected struct and use http_connection::HttpConnection trait. Which pretty much does the same but allows us to be a bit more flexible. I'm honestly in the boat that we should just drop Connect or have it be what https://github.com/tower-rs/tower-http/blob/master/tower-http-util/src/connection.rs#L15 was.