add RTCRtpSender.generateKeyFrame

defined in
  https://w3c.github.io/webrtc-encoded-transform/#rtcrtpsender-extension

Note: this does not implement the "rid(s)" parameter which will be done in a future CL.

VP8 still synchronizes keyframes on all layers even when asked for ones on individual layers while H264 (when implemented as three different encoders in SimulcastEncoderAdapter) can actually utilize this.

This does not change the behavior when receiving a RTCP PLI for a particular layer.

BUG=chromium:1354101

Change-Id: Ic8b14d155242e32c9aeafa55fe6652f346ac76b8
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/274169
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Reviewed-by: Ilya Nikolaevskiy <ilnik@webrtc.org>
Commit-Queue: Philipp Hancke <phancke@microsoft.com>
Cr-Commit-Position: refs/heads/main@{#38472}

api/rtp_sender_interface.h

@@ -104,6 +104,9 @@ class RTC_EXPORT RtpSenderInterface : public rtc::RefCountInterface {
       std::unique_ptr<VideoEncoderFactory::EncoderSelectorInterface>
           encoder_selector) = 0;
 
+  // TODO(crbug.com/1354101): make pure virtual again after Chrome roll.
+  virtual RTCError GenerateKeyFrame() { return RTCError::OK(); }
+
  protected:
   ~RtpSenderInterface() override = default;
 };