Initial attempt at a C++ API for defining channel filters.

BUILD

Makefile

build.yaml

include/grpc++/channel_filter.h

src/cpp/common/channel_filter.cc

tools/doxygen/Doxyfile.c++

tools/doxygen/Doxyfile.c++.internal

tools/run_tests/sources_and_headers.json

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

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

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

...s/vcxproj/grpc++_unsecure/grpc++_unsecure.vcxproj.filters