audioaggregator: Check all downstream allowed caps structures if they support the...

audioaggregator: Check all downstream allowed caps structures if they support the upstream rate

Otherwise it might happen that downstream prefers a different rate (i.e.
puts it into the first structure) and also supports other rates, but
audioaggregator would then fail negotiation.

Also this now correctly handles downstream returning a range of
supported rates.

Fixes https://gitlab.freedesktop.org/gstreamer/gst-plugins-base/-/issues/795

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-base/-/merge_requests/768>