CLI: add-trigger: add --capture option to `on-event` condition

CLI: add-trigger: add --capture option to `on-event` condition

This patch adds a `--capture` option to the `on-event` condition, to
allow specifying the values of payload or context fields to capture.

The filter parser is re-used, as the syntax of the capture expression is
a subset of the filter expression syntax.  Allowed forms are:

  - payload field name: foo
  - context field name: $ctx.foo
  - app-specific field name: $app.foo:bar

After any of these, array indexing can be used.  For example,
`$ctx.foo[2]`.

Change-Id: I6246148634053b32294956d1f7a03f2798fd1d71
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Depends-on: lttng-ust: I5a800fc92e588c2a6a0e26282b0ad5f31c044479