Propagate Sframe options from media channels to the Video RTP send path

WebRtcVideoSendChannel::EnableSframe() and
WebRtcVideoReceiveChannel::EnableSframe() previously only flipped a
local flag with a TODO; nothing below the media channel could observe
it. This CL plumbs the Sframe options from the media channels down to
the video RTP senders.

Changes:

- VideoSendStream::Config gains an SframeSendOptions field and
  VideoReceiveStreamInterface::Config gains an SframeReceiveOptions field.
  Both are populated from the media channel when a stream is created, and
  the receive config copies the field in Config::Copy().
- RtpTransportControllerSendInterface::CreateRtpVideoSender() takes the send
  options and forwards them through RtpVideoSender into
  RTPSenderVideo::Config::sframe_required.
- RTPSenderVideo::SendEncodedImage() fails closed: when Sframe is required
  the frame is dropped instead of being sent in the clear. The encryptor is
  not wired up yet, so today this drops every frame on an Sframe-enabled
  stream.
- WebRtcVideoSendChannel::EnableSframe() pushes the new options to already
  created send streams via WebRtcVideoSendStream::UpdateSframeOptions(),
  which recreates the underlying VideoSendStream. Receive streams still only
  pick the options up at construction; pushing to live receive streams needs
  a setter on VideoReceiveStreamInterface and is left as a TODO.

The audio path is intentionally untouched and will be handled
separately.

Tests: new unit tests cover that Sframe is off by default, that enabling
it before AddSendStream()/AddRecvStream() reaches the stream config, and
that RTPSenderVideo drops encoded images when Sframe is required but no
encryptor is present.

Bug: webrtc:479862368
Change-Id: I40d8298f7a4e4054ef3306dc05d13867a6bf3772
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/506321
Reviewed-by: Tomas Gunnarsson <tommi@webrtc.org>
Commit-Queue: Kacper Wasniowski <kwasniow@cisco.com>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#48782}