Ladybird+LibWeb+WebContent: Create a platform plugin for playing audio

This creates (and installs upon WebContent startup) a platform plugin to
play audio data.

On Serenity, we use AudioServer to play audio over IPC. Unfortunately,
AudioServer is currently coupled with Serenity's audio devices, and thus
cannot be used in Ladybird on Lagom. Instead, we use a Qt audio device
to play the audio, which requires the Qt multimedia package.

While we use Qt to play the audio, note that we can still use LibAudio
to decode the audio data and retrieve samples - we simply send Qt the
raw PCM signals.

Userland/Services/WebContent/AudioCodecPluginSerenity.cpp

@@ -0,0 +1,59 @@
+/*
+ * Copyright (c) 2023, Tim Flynn <trflynn89@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibAudio/ConnectionToServer.h>
+#include <WebContent/AudioCodecPluginSerenity.h>
+
+namespace WebContent {
+
+ErrorOr<NonnullOwnPtr<AudioCodecPluginSerenity>> AudioCodecPluginSerenity::create()
+{
+    auto connection = TRY(Audio::ConnectionToServer::try_create());
+    return adopt_nonnull_own_or_enomem(new (nothrow) AudioCodecPluginSerenity(move(connection)));
+}
+
+AudioCodecPluginSerenity::AudioCodecPluginSerenity(NonnullRefPtr<Audio::ConnectionToServer> connection)
+    : m_connection(move(connection))
+{
+}
+
+AudioCodecPluginSerenity::~AudioCodecPluginSerenity() = default;
+
+size_t AudioCodecPluginSerenity::device_sample_rate()
+{
+    if (!m_device_sample_rate.has_value())
+        m_device_sample_rate = m_connection->get_sample_rate();
+    return *m_device_sample_rate;
+}
+
+void AudioCodecPluginSerenity::enqueue_samples(FixedArray<Audio::Sample> samples)
+{
+    m_connection->async_enqueue(move(samples)).release_value_but_fixme_should_propagate_errors();
+}
+
+size_t AudioCodecPluginSerenity::remaining_samples() const
+{
+    return m_connection->remaining_samples();
+}
+
+void AudioCodecPluginSerenity::resume_playback()
+{
+    m_connection->async_start_playback();
+}
+
+void AudioCodecPluginSerenity::pause_playback()
+{
+    m_connection->async_start_playback();
+}
+
+void AudioCodecPluginSerenity::playback_ended()
+{
+    m_connection->async_pause_playback();
+    m_connection->clear_client_buffer();
+    m_connection->async_clear_buffer();
+}
+
+}