[client] usb audio: emulate UAC2 playback

Add a fixed-format USB Audio Class 2 playback device for the
client-side USB redirection bridge.

Advertise stereo 48 kHz IEEE float audio and handle enumeration,
configuration, alternate settings, clock requests, and isochronous data.

client/CMakeLists.txt

@@ -216,6 +216,7 @@ endif()
 
 if(ENABLE_USB_AUDIO)
   list(APPEND SOURCES
+    src/usb_audio.c
     src/usbredir.c
   )
 endif()