lttng add-context: add --list option

Since --help is replaced by the command's man page, we need
a place to get the list of available context types. The new
--list option of add-context does this.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>