Feature Request: add appendFmtp for m=audio

[leporel]

It would be convenient if it were possible to add not only sdp to video, but also to audio
for example:

const connectionConfig = {
    maxVideoBitrate: 5000,
    sdp: {
        appendFmtp: {
            video: `profile-level-id=4D0029;`,
            audio: `stereo=1;maxaveragebitrate=128000;`
            }
        }
}

or for backward compatibility:

const connectionConfig = {
    maxVideoBitrate: 5000,
    sdp: {
        appendFmtp: `profile-level-id=4D0029;`,
        appendFmtpAudio: `stereo=1;maxaveragebitrate=128000;`
        }
}

[SangwonOh]

@leporel Nice idea. It can be implemented soon. After feature added I will let you know.