LibAudio: Implement PlaybackStream for macOS using Audio Unit framework

https://developer.apple.com/documentation/audiounit

Apple has a number of audio frameworks we could use. This uses the Audio
Unit framework, as it gives us most control over the rendering of the
audio frames (such as being able to quickly pause / discard buffers).
From some reading, we could implement niceties such as fading playback
in and out while seeking over a short (10ms) period. This patch does not
implement such fancy features though.

Co-Authored-By: Andrew Kaster <akaster@serenityos.org>

Userland/Libraries/LibAudio/PlaybackStreamAudioUnit.h

@@ -0,0 +1,39 @@
+/*
+ * Copyright (c) 2023, Andrew Kaster <akaster@serenityos.org>
+ * Copyright (c) 2023, Tim Flynn <trflynn89@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Error.h>
+#include <AK/NonnullRefPtr.h>
+#include <LibAudio/PlaybackStream.h>
+
+namespace Audio {
+
+class AudioState;
+
+class PlaybackStreamAudioUnit final : public PlaybackStream {
+public:
+    static ErrorOr<NonnullRefPtr<PlaybackStream>> create(OutputState initial_output_state, u32 sample_rate, u8 channels, u32 target_latency_ms, AudioDataRequestCallback&& data_request_callback);
+
+    virtual void set_underrun_callback(Function<void()>) override;
+
+    virtual NonnullRefPtr<Core::ThreadedPromise<Duration>> resume() override;
+    virtual NonnullRefPtr<Core::ThreadedPromise<void>> drain_buffer_and_suspend() override;
+    virtual NonnullRefPtr<Core::ThreadedPromise<void>> discard_buffer_and_suspend() override;
+
+    virtual ErrorOr<Duration> total_time_played() override;
+
+    virtual NonnullRefPtr<Core::ThreadedPromise<void>> set_volume(double) override;
+
+private:
+    explicit PlaybackStreamAudioUnit(NonnullRefPtr<AudioState>);
+    ~PlaybackStreamAudioUnit();
+
+    NonnullRefPtr<AudioState> m_state;
+};
+
+}