Call gst_aggregator_selected_samples() after identifying the
caption buffers that will be added as a meta on the next video
buffer.
Implement GstAggregator.peek_next_sample.
Add an example that demonstrates usage of the new API in
combination with the existing buffer-consumed signal.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-bad/-/merge_requests/1390>