Support percent-encoding status messages

include/grpc/support/slice.h

src/core/lib/channel/http_client_filter.c

src/core/lib/channel/http_server_filter.c

src/core/lib/support/slice.c

src/core/lib/surface/call.c

src/core/lib/transport/metadata_batch.c

test/core/end2end/tests/binary_metadata.c