Use zstandard implementation from stdlib (PEP-784)

[Rogdham]

Summary

Context

Thanks to PEP-784, Zstandard is included in Python starting from version 3.14 with the compression.zstd module.

So for Python 3.14+, we don't need an external lib. For older version of Python, I'm using backports.zstd which exposes the same API as stdlib.

This also means that users of Python 3.14+ will benefit from Zstandard support even if they install httpx instead of httpx[zstd].

A similar change has been adopted in various other libraries already, such as urllib3 from version 2.6.0.

Full disclosure: I'm the author and maintainer of backports.zstd, and the maintainer of pyzstd (which code was used as a base for the integration into Python). I also helped with PEP-784 and its integration into CPython.

Checklist

• I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.

[Rogdham]

Hello @mbeijen, as the author of Zstandard support in httpx your review would be appreciated! 🤗

[mbeijen]

Thanks for your work, this looks great (esp. together with #3726 !!!)