Add WebCodecs-Echo to the samples directory

[aboba]

Rebase of #415

Live demo: https://webrtc.internaut.com/wc/wtSender10/

Requires Chromium M108+ (to support BYOB reader)

Known issues:

1. Device selection (different behavior in Chrome vs. Chrome Canary).
2. FIXED: Non-optimal write strategy (added await writer.ready, removed await from writer.write, writer.close)
3. Server support for "cut-through" forwarding
4. Server support for RESET_STREAM forwarding
5. PARTIALLY FIXED: Non-optimal read strategy (removed awaits on read side, while {} loop).
6. FIXED: Add writer.releaseLock() and reader.releaseLock()

[aboba]

Recent updates:

1. Created an async writeChunk() function, removing await in writer.write(chunk) and writer.close(). This appears to decrease glass-glass latency substantially and reordering is now observed on the receiver (requiring a reordering buffer).

2. Removed some awaits in reader.read(). This also appears to have enabled concurrency and re-ordering.

[ricea]

3. Tried using writer.close().then() instead of await writer.close(). This resulted in much more reordering, but also frame RTTs that grew over time to very high levels (2000+ ms), causing many more aborted writes. Not exactly sure why this change made so much difference.

This really shouldn't make any difference. Very long RTTs would seem to imply that backpressure is not working effectively.

[aboba]

@ricea Wrote an async writeChunk() function, and made sure to call writer.releaseLock() eventually on all writers, and reader.releaseLock() on all readers. The strange issues with writer.close().then() causing huge glass-glass latency compared with await writer.close() seem to have vanished.