[Feature Request] Support JSON Type Converters

[cretz]

Is your feature request related to a problem? Please describe.

People need to customize temporal.converters.value_to_type for use with their types during JSON conversion

Describe the solution you'd like

Add a JSONTypeConverter ABC that converts to and from a user type to JSON-able type. Change JSONPlainPayloadConverter to accept a collection of these and make that easier to set in construction of default payload converter.

[michael-cybrid]

Big 👍 from us on this.

We have conversion working OpenAPI generated models working by inheriting from JSONPlainPayloadConverter and doing something like the following but it has limitations (around optional and collections, etc.) and first party support would be awesome:

 def from_payload(
        self,
        payload: Payload,
        type_hint: Type[Any] | None = None,
    ) -> Any:
        """See base class."""
        try:
            obj = json.loads(payload.data, cls=self._decoder)
            if type_hint:
                if service_one.is_openapi_model_subclass(type_hint):
                    obj = service_one.dict_to_model(obj, type_hint)

                elif service_two.is_openapi_model_subclass(type_hint):
                    obj = service_two.dict_to_model(obj, type_hint)

                else:
                    obj = value_to_type(type_hint, obj)
            return obj
        except json.JSONDecodeError as err:  # pragma: no cover
            raise RuntimeError("Failed parsing") from err

[cretz]

it has limitations (around optional and collections, etc.)

@michael-cybrid - I think for custom input/output types it can be clearer to use your own encoding payload converter with your own encoding (e.g. json/open-api) instead of reusing the existing encoding. This makes it more portable/obvious to other Temporal clients/users that custom logic is needed. Granted I understand this doesn't help with collections and such, but usually you know which types you need to support for your own conversion.

Regardless, I have just opened a PR for JSON type converters and README updates that can help your use case if reusing existing JSON conversion logic.