CLI: make list-triggers command print capture expressions

When printing an event rule condition, in the `list-triggers` command,
print the capture expressions associated with that condition.

The result looks like:

    - id: T1
      condition: event rule hit
        rule: allo (type: tracepoint, domain: ust)
        captures:
          - $ctx.foo
      actions:
        notify
    - id: T2
      condition: event rule hit
        rule: allo (type: tracepoint, domain: ust)
        captures:
          - $app.iga:active_clients
      actions:
        notify

Change-Id: Iaa902369a2df3edf2928a935f7565a7c21acb861
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Depends-on: lttng-ust: I8423c510bf6af2f9bf85256e8d6f931d36f7054b