Exposed channel target and call peer in PHP

src/php/ext/grpc/call.c

src/php/ext/grpc/channel.c

src/php/lib/Grpc/AbstractCall.php

src/php/lib/Grpc/BaseStub.php

src/php/tests/generated_code/AbstractGeneratedCodeTest.php

src/php/tests/interop/interop_client.php

src/php/tests/unit_tests/CallTest.php

src/php/tests/unit_tests/EndToEndTest.php