Add C++ support for using external FD

BUILD

Makefile

build.yaml

include/grpc++/create_channel_posix.h

include/grpc++/grpc++.h

include/grpc++/server.h

include/grpc++/server_posix.h

include/grpc/grpc_posix.h

include/grpc/impl/codegen/port_platform.h

...t/transport/chttp2/client/insecure/channel_create_posix.c

...xt/transport/chttp2/server/insecure/server_chttp2_posix.c

src/cpp/client/create_channel_posix.cc

src/cpp/server/server.cc

src/cpp/server/server_posix.cc

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

src/ruby/ext/grpc/rb_grpc_imports.generated.h

tools/doxygen/Doxyfile.c++

tools/doxygen/Doxyfile.c++.internal

tools/run_tests/sources_and_headers.json

vsprojects/vcxproj/grpc++/grpc++.vcxproj