rust: change client::*::from_stub() to accept Into<Arc<T>>

This issue serves https://github.com/googleapis/google-cloud-rust/issues/2146

This is a non-breaking change to make the `from_stub` constructor for client. We can make it more usable if it accepts `Into<Arc<T>>`.