Support custom JSON serializers

[0x12th]

Adds per-client and per-request JSON serializer and deserializer hooks so callers can use libraries such as orjson. This keeps the existing default JSON behavior unchanged while allowing request bodies and response decoding to be customized independently.

Closes #259.

[Kludex]

I agree with the maintainer's position in the issue that motivated this PR: I don't think this is the API shape that we want to achieve.

I agree there's a problem, and I'm happy to engage in a discussion about it. Also, I'm not that sure we should treat JSON deserialization (response) as the same "thing" as JSON serialization.

[0x12th]

I agree with the maintainer's position in the issue that motivated this PR: I don't think this is the API shape that we want to achieve.

I agree there's a problem, and I'm happy to engage in a discussion about it. Also, I'm not that sure we should treat JSON deserialization (response) as the same "thing" as JSON serialization.

Thanks, that makes sense.

I agree that request-side JSON serialization and response-side JSON decoding probably should not be treated as the same concern. I’m going to rework the PR into a smaller proposal focused only on configurable response JSON decoding, with a reduced API surface and semantics closer to the original issue discussion.

[Kludex]

I think it's better if you can create a discussion first with the proposal, so we can discuss in there. Would that okay for you?

[0x12th]

I think it's better if you can create a discussion first with the proposal, so we can discuss in there. Would that okay for you?

Yes, okay.