bt2: TraceCollectionNotificationIterator: support custom filter CCs

This patch adds an optional `filter_component_specs` parameter to
TraceCollectionNotificationIterator's constructor so that the trace
collection notification iterator supports a chain of custom filter
components. The filter chain is connected to the implicit muxer's output
port or to the implicit trimmer's output port if it exists.

This is useful to add debugging information, for example:

    src = bt2.ComponentSpec('ctf', 'fs', trace_path)
    flt = bt2.ComponentSpec('lttng-utils', 'debug-info')
    it = bt2.TraceCollectionNotificationIterator(src, flt)

    for notif in it:
        ...

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