fix: slow rampup on vp9/h264 codec

[santhoshvai]

💡 Overview

Adds an optional way to increase the start bitrate of the av1/vp9/h264 codec video encoder

📝 Testing

1. Enable webrtc logs in iOS

    let options = WebRTCModuleOptions.sharedInstance()
    options.loggingSeverity = .info

2. Create call with

        call.updatePublishOptions({
          dangerouslySetStartBitrateFactor: 0.7,
        });
        call.getOrCreate()

3. Join a call on pronto web

4. Filter iOS logs in xcode with SetStartBitrate and you should see
   (video_stream_encoder.cc:914): SetStartBitrate 1050000000
   (assuming pronto web uses vp9 1500kbps)