Cannot do anything without `native-client` feature

[pbzweihander]

Since clients are all pub(crate) and one_off module is depend on native-client feature, we can't do anything without native-client feature. We should make other clients public or used by one_off module.

[yoshuawuyts]

There's a private with_client method right now (doc hidden, publicly accessible tho) that will be opened up at some point. Wanted to make sure we indeed got the http-client interfaces right, but I think they're looking quite good which means we'll be able to expose them in the near future (and in turn make surf useful without native-client enabled.)