Exposing the number of bytes available to read

There's currently no way to determine how many bytes can be returned by `stream_recv`.

I'm currently writing an async API on top of quiche and unfortunately, that means allocating on the heap via `Vec` or `BytesMut`. With no way of knowing the size upfront, I either have to over-allocate or perform an additional copy (using a temporary buffer to learn the size).

Something like `stream_capacity` but in the reverse direction would be great. `stream_buffered` maybe?

Start by locating the `stream_recv` and `stream_capacity` APIs in quiche and compare how they expose readable and writable stream state. Determine the intended meaning of a reverse-capacity method for async callers using `Vec` or `BytesMut`; done means the API reports the available readable byte count without requiring a temporary read or extra copy, with corresponding tests if the existing API area has them.

Written by the indexing model from the issue text.