Add support for custom JSON encoders/decoders.

[AndreCimander]

Summary

Added customizable JSON encoder/decoder.

Working example (also provided in docs):

import httpx
import msgspec
import typing


encoder = msgspec.json.Encoder()
def custom_json_encoder(json_data: typing.Any) -> bytes:
    return encoder.encode(json_data)

httpx.register_json_encoder(custom_json_encoder)


decoder = msgspec.json.Decoder()
def custom_json_decoder(json_data: bytes, **kwargs: Any) -> bytes:
    return decoder.decode(json_data)

httpx.register_json_decoder(custom_json_decoder)

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.

[AndreCimander]

It's getting late over here, still need to figure out how to satisfy mypy with Protocol usage, as mypy_extension.KwArg and mypy_extenions.Arg are deprecated and I don't want to import those (even with if TYPE_CHECKING: import guard).

[AndreCimander]

After wrangling with correct type definitions for **kwargs, I concede defeat and decided to use # type: ignore.

If there is a better solution for this I'd would be more than happy to learn, this is a little frustrating :)