Split codegen target and made it libs

BUILD

Makefile

binding.gyp

build.yaml

gRPC.podspec

grpc.gemspec

package.json

test/core/surface/public_headers_must_be_c89.c

tools/doxygen/Doxyfile.c++

tools/doxygen/Doxyfile.c++.internal

tools/doxygen/Doxyfile.core

tools/doxygen/Doxyfile.core.internal

tools/run_tests/sources_and_headers.json

vsprojects/buildtests_c.sln

vsprojects/grpc.sln

vsprojects/grpc_csharp_ext.sln

vsprojects/grpc_protoc_plugins.sln

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

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

...cts/vcxproj/grpc++_codegen_lib/grpc++_codegen_lib.vcxproj