[client] audio: add transport provider interface

Route audio through a provider interface. Connected transports take
priority while SPICE remains the fallback.

Carry explicit sample formats, rates, channel layouts, and source clocks
through playback and recording. Add optional presentation clock feedback
for active synchronization.

client/CMakeLists.txt

@@ -188,7 +188,10 @@ if(CMAKE_C_COMPILER_ID MATCHES "GNU|Clang")
 endif()
 
 if(ENABLE_AUDIO)
-  list(APPEND SOURCES src/audio.c)
+  list(APPEND SOURCES
+    src/audio.c
+    src/audio_spice.c
+  )
 endif()
 
 add_subdirectory("${PROJECT_TOP}/resources"       "${CMAKE_BINARY_DIR}/resources")