Refactor encoder-complexity param in VideoCodec w/backward compatibility

Move complexity parameter to the main VideoCodec class to enable
additional video codecs to use the parameter without creating a new
codec-specific structure.

Bug: webrtc:13694
Change-Id: Icb7cf640b178875d799f39ade8b5084e3222bb1c
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/251921
Reviewed-by: Erik Språng <sprang@webrtc.org>
Commit-Queue: Michael Horowitz <mhoro@google.com>
Cr-Commit-Position: refs/heads/main@{#36040}

modules/video_coding/codecs/vp8/libvpx_vp8_encoder.cc

@@ -591,7 +591,7 @@ int LibvpxVp8Encoder::InitEncode(const VideoCodec* inst,
   }
 
   // Allow the user to set the complexity for the base stream.
-  switch (inst->VP8().complexity) {
+  switch (inst->GetVideoEncoderComplexity()) {
     case VideoCodecComplexity::kComplexityHigh:
       cpu_speed_[0] = -5;
       break;