v4l2videoenc: replace custom QUERY_CAPS handling with getcaps callback

v4l2videoenc: replace custom QUERY_CAPS handling with getcaps callback

The videoencoder base class uses getcaps() to ask a subclass for the caps in its
sink_query_default() implementation.

Replace the custom handling of the QUERY_CAPS in the v4l2videoenc with an
implementation of getcaps() that returns the caps that are supported by the
v4l2videoenc to return these caps in the query.

This getcaps() implementation also calls the provided proxy_getcaps(), which
sends a caps query to downstream. This fixes the v4l2videoenc element to respect
limits of downstream elements in a sink query.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/5036>