Add individual rate adjustment per substream for Opus

[thirv]

This is an experimental tool that is useful for sound quality optimization. Open to suggestions about the UI etc. Also, not sure if this would somehow be a hindrance and is better left out.

Usage example in a .textproto config file:

codec_config_metadata {
  codec_config_id: 200
  codec_config {
    codec_id: CODEC_ID_OPUS
    num_samples_per_frame: 960
    audio_roll_distance: -4
    decoder_config_opus {
      version: 1
      pre_skip: 312
      input_sample_rate: 48000
      opus_encoder_metadata {
        target_bitrate_per_channel: 16000
        application: APPLICATION_AUDIO
        use_float_api: false
        use_substream_rate_override: true
        substream_rate_overrides: [64000, 57000, 30000, 9000]
      }
    }
  }
}