lttng-ctl: add capture descriptor feature to event rule condition API

lttng-ctl: add capture descriptor feature to event rule condition API

This patch makes it possible to add capture descriptors to an event rule
condition.

A capture descriptor describes a value to be captured by an LTTng tracer
when an event occurs and to be attached to the corresponding condition's
evaluation.

The lttng_condition_event_rule_append_capture_descriptor() function
appends a capture descriptor to an event rule condition. The capture
descriptor is an event expression object of which the ownership is moved
to the condition object. The verb "append" here indicates that the order
of the condition's capture descriptors is significant: indeed, when
liblttng-ctl gets an upgrade to the "event rule hit" condition
evaluation API in the future to make its user read the captured values,
such values will be ordered the same way, therefore sharing the same
numeric indexes.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I68f163e6eb011e690f648d9ff8ea3eb89af004ff
Depends-on: lttng-ust: I5a800fc92e588c2a6a0e26282b0ad5f31c044479