Add read_timeout option

[seanmonstar]

The ClientBuilder::timeout() option describes a timeout for the entire request and response. Also considered a deadline. So, if you set the timeout to 30 seconds, if the full response hasn't finished after 30s, the streaming will error. However, many times that's not what is wanted. Many would like a timeout to detect a stalled download, but can't know ahead of time how big the download is.

Solution: add ClientBuilder::read_timeout(Duration), which is a recurring timeout for each read operation. Basically, if you set the read_timeout to 30s, it will wait up to 30s for the response headers. Then, each chunk of the body gets a new slice of 30 seconds.

Implementation wise, this can make use of the TimeoutBody type in tower-http, internally inside Response, if the timeout is set.