[Feature Request] Support using system versions of gRPC, Protobuf, Abseil in CMake

[prm-james-hill]

The existing CMake is hard to hack around. It fetches a bunch of other Triton repos which unconditionally try to set up dependencies. Even if you only build the HTTP client it from source it still tries to set up dependencies which it may not need like re2 which will conflict with earlier CMake.