Build Issue: Can't build with RAPIDJSON_SYS_DEP=OFF

[peterychang]

When the build option RAPIDJSON_SYS_DEP=OFF, the clientlib will attempt to pull the version used from ext_libs/vcpkg. VW, however will attempt to pull it from ext_libs/vowpal_wabbit/ext_libs/rapidjson.

The version used in the clientlib is based on the commit in vcpkg while the version in VW is at 1.1.0. This causes a version mismatch which will cause linker failures.

This problem can't be solved by simply overriding the version in vcpkg.json either, since doing so will cause a different error when RAPIDJSON_SYS_DEP=ON; namely this line will fail with the error Could not find a package configuration file provided by "RapidJSON"

A short term fix would be to make VW use vcpkg for rapidjson regardless of the SYS_DEP flag. A more thorough fix would be to refactor the build so the clientlib take all the common dependencies from VW