Add shim for Python's gRPC core on Windows

setup.py

src/python/grpcio/grpc/_cython/.gitignore

src/python/grpcio/grpc/_cython/_cygrpc/grpc.pxi

src/python/grpcio/grpc/_cython/_windows/grpc.def

src/python/grpcio/grpc/_cython/cygrpc.pyx

src/python/grpcio/grpc/_cython/imports.generated.c

src/python/grpcio/grpc/_cython/imports.generated.h

src/python/grpcio/grpc/_cython/loader.c

src/python/grpcio/grpc/_cython/loader.h

...c/python/grpcio/grpc/_cython/imports.generated.c.template

...c/python/grpcio/grpc/_cython/imports.generated.h.template