Add/Expose loudnessDb for audio-only streams

[anandnet]

Is your feature request related to a problem? Please describe.

Could you please expose/add loudnessDb for audio-only streams?

 {
                "itag": 140,
                "mimeType": "audio/mp4; codecs=\"mp4a.40.2\"",
                "bitrate": 130600,
                "initRange": {
                    "start": "0",
                    "end": "631"
                },
                "indexRange": {
                    "start": "632",
                    "end": "891"
                },
                "lastModified": "1705711430651037",
                "contentLength": "2963461",
                "quality": "tiny",
                "projectionType": "RECTANGULAR",
                "averageBitrate": 129503,
                "highReplication": true,
                "audioQuality": "AUDIO_QUALITY_MEDIUM",
                "approxDurationMs": "183066",
                "audioSampleRate": "44100",
                "audioChannels": 2,
                "loudnessDb": -0.42000008,
                "signatureCipher": "
  },