Allow overriding the default JSON encoder library

[radupotop]

I'm in the process of migrating from the build-in json encoder library to the much faster Orjson library, in a rather large project. One of the approaches I've been looking at is to monkey-patch the requests library, so that I don't have to rewrite all the handlers.

This is easier said than done unfortunately. So I'm now looking at httpx, but unfortunately I would still have to monkey-patch the _main and the _models modules.

It would be great if there way a way to specify custom dumps and loads handlers for JSON.

For example Pydantic supports this using the json_loads and json_dumps properties in its Config: https://docs.pydantic.dev/usage/exporting_models/#custom-json-deserialisation

[Kludex]

https://github.com/encode/httpx/issues/717