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 :)

[T-256]

Also, see #717 and its linked pr: #3199

[lovelydinosaur]

Appreciate that there's clearly demand for something like this, tho have been clear that a global registry approach isn't going to be accepted. Can we close this off, and continue pushing for design-discussion-first approachs?

[AndreCimander]

Ah, sorry, haven't seen that comment, should have searched more diligently.

Bummer, well, yeah, I can understand your point. But I really would have favored a global registry call in my projects :)

A customized Client solution is fine as well (but it feels a bit off needing to customize 3 classes just for a simple json callable change).

I'll close this PR then.

[lovelydinosaur]

Thanks @AndreCimander this one is proving a bit of a thorny issue.

[gtors]

A customized Client solution is fine as well (but it feels a bit off needing to customize 3 classes just for a simple json callable change).

@AndreCimander Try httpj 😎 It's my fork with similar functionality:

import httpj
import msgspec
import typing


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

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

client = httpj.Client(json_serialize=custom_json_encoder, json_deserialize=custom_json_decoder)