Support compression.zstd in Python 3.14

[Zaczero]

Python will now support zstd compression in the standard library. Users installing HTTPX in Python 3.14 or newer won't need to install the zstandard third-party extension.

https://docs.python.org/3.14/whatsnew/3.14.html#pep-784-adding-zstandard-to-the-standard-library

[mollymorphous]

I think this would be great to have! I took a shot at implementing it in #3613