MEDIUM: cli: Add payload support

MEDIUM: cli: Add payload support

In order to use arbitrary data in the CLI (multiple lines or group of words
that must be considered as a whole, for example), it is now possible to add a
payload to the commands. To do so, the first line needs to end with a special
pattern: <<\n. Everything that follows will be left untouched by the CLI parser
and will be passed to the commands parsers.

Per-command support will need to be added to take advantage of this
feature.

Signed-off-by: Aurélien Nephtali <aurelien.nephtali@corp.ovh.com>