set `x-google-start-bitrate` for video tracks using SVC codecs, to improve initial video resolution

[davidzhao]

libwebrtc has been observed to ramp up bitrate when publishing VP9 or AV1 tracks. This behavior could result in videos published to start with a lower resolution.

In our JS SDK, there's logic to specifically guide the start bitrate to ensure higher resolution to start. Given our Rust SDK is being used increasingly for video publishing , we should handle this behavior automatically.