Add MixingAudioDeviceModule with loopback mixing, mic muting and playback volume control

Introduces MixingAudioDeviceModule and MixingAudioControl to mix
system-audio loopback into the outgoing microphone stream, plus two
runtime controls for mic-only muting and per-call playback volume.

MixingAudioDeviceModule (ADM wrapper):
  Installs MixingAudioTransport in RegisterAudioCallback(),
  starts/stops DirectLoopbackCapture (Linux) or a dedicated loopback
  ADM + LoopbackAdmTransport (Windows) when setLoopbackEnabled() is
  toggled via MixingAudioControl.

MixingAudioTransport:
  setMicrophoneMuted(bool) -- zeroes mic samples before forwarding
    to WebRTC while keeping the channel open so loopback audio
    continues to flow through unchanged.
  setPlaybackVolume(float) -- scales decoded playback samples by the
    given factor (0.0-1.0), providing per-call software volume
    control independent of the system mixer.

MixingAudioControl (public API):
  Shared handle that survives ADM recreation. setLoopbackEnabled(),
  setMicrophoneMuted() and setPlaybackVolume() let callers control
  mixing at any time; state is re-applied whenever a new
  MixingAudioDeviceModule attaches.

MixingAudioDeviceModuleCreator(innerCreator, control):
  Returns a creator lambda that wraps any ADM inside a
  MixingAudioDeviceModule.

Platform: Linux and Windows. macOS is unaffected.

Author: paterkleomenis