Write a HIP for Body::poll_progress() #4228

hyperium/http-body#90 proposes the addition of a new method to the http_body::Body trait. because Body is a foundational interface used in HTTP clients and servers, altering this trait is a significant change that would benefit from some consideration.

#4131 introduced an RFC-like process to hyper, to help provide a mechanism for proposing non-trivial changes like this.

the benefits of a poll_progress() method have come up in various discussions since then, and a call for comments was posted a little over a year ago.

to help move this proposal forward, let's digest feedback we received, and restate the case for this in a HIP.