Generate C++ Apis

[TheOneRing]

[labkode]

@TheOneRing the GRPC definitions needs to be compiled in the toolchain used by the ownCloud desktop client as they are very coupled to the target build system you build the sync client.

To generate for subfolders you can find some examples here for JS:

https://github.com/cs3org/cs3apis-build/blob/master/build.go#L382

You then need to compile grpc in your machine: https://github.com/grpc/grpc/blob/master/src/cpp/README.md#make

and follow that tutorial.