codecs: h264decoder: Add support for output delay

codecs: h264decoder: Add support for output delay

Some decoding APIs support delayed output or a command for decoding
a frame doesn't need to be sequential to corresponding command for
getting decoded frame. For instance, subclass might be able to
request decoding for multiple frames and then get for one (oldest)
decoded frame or so.
If aforementioned case is supported by specific decoding API,
delayed output might show better throughput performance.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-bad/-/merge_requests/1925>