php: rename credentials to channel_credentials

src/php/ext/grpc/call_credentials.c

src/php/ext/grpc/channel.c

src/php/ext/grpc/channel_credentials.c

src/php/ext/grpc/channel_credentials.h

src/php/ext/grpc/config.m4

src/php/ext/grpc/package.xml

src/php/ext/grpc/php_grpc.c