Add support for OnFrameDropped callback in FrameEncodeMetadataWriter.

This wires the new OnFrameDropped callback method up through the encoder
metadata writer. Similar to SimulcastEncoderAdapter, it attempts to
detect gaps in the callbacks to detect frame drops - so that we can
support legacy encoders in a best-effort manner. Down the line the new
behavior should however be made mandatory.

Bug: webrtc:467444018
Change-Id: I8436fa64f8b2f6286008ebf0183188c0d279d1de
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/448700
Auto-Submit: Erik Språng <sprang@webrtc.org>
Commit-Queue: Sergey Silkin <ssilkin@webrtc.org>
Reviewed-by: Sergey Silkin <ssilkin@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#46895}