Add support for zstd decoding (#3139)

Add support for zstd decoding (#3139)

This adds support for zstd decoding using the python package zstandard.
This is similar to how it is implemented in urllib3. I also chose the
optional installation option httpx[zstd] to mimic the same option in
urllib3.

zstd decoding is similar to brotli, but in benchmarks it is supposed to
be even faster. The zstd compression is described in RFC 8878.

See https://github.com/encode/httpx/discussions/1986

Co-authored-by: Kamil Monicz <kamil@monicz.dev>