Change which impl the cpp conformance testee uses for JSON parse/serialize.

The Cpp JSON implementation has two separate paths: one that uses type.proto and one which uses classic descriptor pool. These two cases have two different implementations with some different edge case behaviors, and the Descriptor-based one is the dominant usecase so it makes more sense for us to be hooked up to that one.

This will also allow for extensions to be conformance tested, which is a case that works on this implementation but not the type.proto one (since type.proto doesn't represent extensions).

This CL deliberately does not turn on the `allow_legacy_nonconformant_behavior = false` setting for the testee, to keep the testee running the behavior that nearly all of the C++Proto JSON users are using.

PiperOrigin-RevId: 800519012