SpiceAgent: Let's start rewriting the messaging system :^)

The old message system was very dependent on syscalls, and the overall
structure made it hard to implement new features.

The new message system is pretty expandible, where each message has its
own dedicated class. As well as this, we now use Core::File and
AK::Stream for reading and writing messages.

Using AK::Stream also allows us to change the actual data source
(in this case, Core::File) without having to update a whole lot of code
in the future.

Userland/Services/SpiceAgent/MessageHeader.h

@@ -0,0 +1,65 @@
+/*
+ * Copyright (c) 2023, Caoimhe Byrne <caoimhebyrne06@gmail.com>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include "Message.h"
+#include <AK/ByteBuffer.h>
+#include <AK/Format.h>
+#include <AK/Forward.h>
+
+namespace SpiceAgent {
+
+// An incoming or outgoing message header.
+// This contains information about the message, like how long it is, the type, etc.
+class [[gnu::packed]] MessageHeader {
+public:
+    MessageHeader(Message::Type type, u32 data_size, u32 protocol_version = AGENT_PROTOCOL, u64 opaque = 0)
+        : m_protocol_version(protocol_version)
+        , m_type(type)
+        , m_opaque(opaque)
+        , m_data_size(data_size)
+    {
+    }
+
+    Message::Type type() const { return m_type; };
+    u32 data_size() const { return m_data_size; };
+    u32 protocol_version() const { return m_protocol_version; };
+    u64 opaque() const { return m_opaque; };
+
+private:
+    // The protocol version being used.
+    u32 m_protocol_version { AGENT_PROTOCOL };
+
+    // The message type present in `data`.
+    Message::Type m_type { Message::Type::MouseState };
+
+    // A placeholder for message types which only need to pass a single integer as message data,
+    // for message types which have more data it is always set to 0.
+    u64 m_opaque { 0 };
+
+    // The size of the data in the message following this header.
+    u32 m_data_size { 0 };
+};
+
+}
+
+template<>
+struct AK::Traits<SpiceAgent::MessageHeader> : public AK::GenericTraits<SpiceAgent::MessageHeader> {
+    static constexpr bool is_trivially_serializable() { return true; }
+};
+
+namespace AK {
+template<>
+struct Formatter<SpiceAgent::MessageHeader> : Formatter<FormatString> {
+    ErrorOr<void> format(FormatBuilder& builder, SpiceAgent::MessageHeader const& header)
+    {
+        return Formatter<FormatString>::format(builder,
+            "MessageHeader {{ protocol_version = {}, type = {}, opaque = {}, data_size = {} }}"sv,
+            header.protocol_version(), to_underlying(header.type()), header.opaque(), header.data_size());
+    }
+};
+}