aggregator: expose sample selection API

aggregator: expose sample selection API

See https://gitlab.freedesktop.org/gstreamer/gst-plugins-base/-/issues/771
for context.

This exposes new API that subclasses must call from their
aggregate() implementation to signal that they have selected
the next samples they will aggregate: gst_aggregator_selected_samples()

GstAggregator will emit a new signal there, `samples-selected`,
handlers can then look up samples per pad with the newly-added
gst_aggregator_peek_next_sample.

In addition, a new FIXME is logged when subclasses haven't actually
called `selected_samples` from their aggregate() implementation.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/549>