Simplify type system to leverage 'Sync' types

These are generally available now that we use hyper's async
APIs along with the most recent yup-hypermock.

src/rust/api/client.rs

@@ -607,7 +607,7 @@ impl RangeResponseHeader {
 
 /// A utility type to perform a resumable upload from start to end.
 pub struct ResumableUploadHelper<'a, A: 'a> {
-    pub client: &'a mut hyper::client::Client<
+    pub client: &'a hyper::client::Client<
         hyper_rustls::HttpsConnector<hyper::client::connect::HttpConnector>,
         hyper::body::Body,
     >,