client: Add a SendStdin API
client: Add a SendStdin API

That's useful only in tests really at this point as the shim is written
in C. But one never knows!

Signed-off-by: Damien Lespiau <64bd3cb94f359c1a3ce68dae5e26b40578526277@intel.com>

clearcontainers/proxy,clearcontainers/proxy