Unmarshal JSON to dynamic protobuf for basic types

Build an internal parsing engine that recursively loops over `serde_json::Value` inputs. It must look up the keys in the dynamic `prost_reflect::DescriptorPool` and cast values onto a `DynamicMessage`.

- [ ] Flat scalars: `String`, `Bool`, `Integer`, `Float`.
- [ ] Nested logic: JSON arrays (mapping to protobuf `Repeated` fields) and nested JSON dictionaries (mapping to protobuf nested `Message` fields internally)