BYO JSON library

The library currently only supports three JSON backends and won't compile without one; this makes it difficult when we have our own JSON library but want to provide a serializer implementation using our own whilst still using submodules.

I can currently work around this by using a copy of `json_serializer.h` with my custom types, but would it be feasible to edit that file to allow passing in custom type names? like if `CPPDAP_BYO_JSON_DESERIALIZER` is set, it just does `using Deserialier = CPPDAP_BYO_JSON_DESERIALIZER;` or something.