Change JSON format to use the standard proto3 mapping for each type.

src/core/ext/client_channel/client_channel.c

src/core/lib/channel/message_size_filter.c

src/core/lib/support/string.c

src/core/lib/support/string.h

src/core/lib/transport/service_config.c

test/core/end2end/connection_refused_test.c

test/core/end2end/tests/cancel_after_accept.c

test/core/end2end/tests/max_message_length.c