Add Per-App Audio Streaming for Virtual Display in Scrcpy

[Shrey113]

Is your feature request related to a problem? Please describe.
Scrcpy supports virtual display (--new-display) for app screen mirroring 🎥, but there’s no built-in way to stream only a specific app’s audio 🔊.

On Android 15, native per-app screen casting with audio is supported without ADB. Adding this to scrcpy would make it even more powerful! 🚀

Describe the solution you'd like
A new option like:
--vd-audio="{app package name || app name}"
This would allow users to stream only a specific app’s audio alongside its screen, keeping it separate from system sounds. 🎧

Additional context
This would be huge for streaming, testing, and content creation! 🎬🎮 Hope to see this added soon! 🙌

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Thank you for creating such an amazing tool—scrcpy has been a game-changer! 🙌

[rom1v]

(refs #5370 (comment))