split iterator into its own file

BUILD

Makefile

build.json

include/grpc++/auth_context.h

include/grpc++/auth_property_iterator.h

src/cpp/common/auth_property_iterator.cc

src/cpp/common/secure_auth_context.cc

src/cpp/common/secure_auth_context.h

test/cpp/common/auth_property_iterator_test.cc

test/cpp/common/secure_auth_context_test.cc

tools/doxygen/Doxyfile.c++

tools/doxygen/Doxyfile.c++.internal

tools/run_tests/sources_and_headers.json

tools/run_tests/tests.json

vsprojects/grpc++/grpc++.vcxproj

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

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

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