Let's have an approach similar to our timeouts to enforce network rate limiting.

Design questions here include "what specifiers should we support" and "what is the API for specifying upload vs. download rates"?

with httpx.rate("10MB/s"):
    ...

with httpx.rate_limit(upload="10MB/s", download="25MB/s"):
    ...

There's also potentially(?) implementation questions here around windowing and smoothing, tho a hard cap might be reasonable. What good prior art is there for us to reference here?