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/Message.cpp

@@ -0,0 +1,55 @@
+/*
+ * Copyright (c) 2023, Caoimhe Byrne <caoimhebyrne06@gmail.com>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include "Message.h"
+#include <AK/MemoryStream.h>
+#include <AK/Stream.h>
+#include <AK/String.h>
+
+namespace SpiceAgent {
+
+ErrorOr<AnnounceCapabilitiesMessage> AnnounceCapabilitiesMessage::read_from_stream(AK::Stream& stream)
+{
+    // If this message is a capabilities request, we don't have to parse anything else.
+    auto is_requesting = TRY(stream.read_value<u32>()) == 1;
+    if (is_requesting) {
+        return AnnounceCapabilitiesMessage(is_requesting);
+    }
+
+    return Error::from_string_literal("Unexpected non-requesting announce capabilities message received!");
+}
+
+ErrorOr<void> AnnounceCapabilitiesMessage::write_to_stream(AK::Stream& stream)
+{
+    TRY(stream.write_value<u32>(is_request()));
+
+    // Each bit in this u32 indicates if a certain capability is enabled or not.
+    u32 capabilities_bits = 0;
+    for (auto capability : capabilities()) {
+        // FIXME: At the moment, we only support up to 32 capabilities as the Spice protocol
+        //        only contains 17 capabilities.
+        auto capability_value = to_underlying(capability);
+        VERIFY(capability_value < 32);
+
+        capabilities_bits |= 1 << capability_value;
+    }
+
+    TRY(stream.write_value(capabilities_bits));
+
+    return {};
+}
+
+ErrorOr<String> AnnounceCapabilitiesMessage::debug_description()
+{
+    StringBuilder builder;
+    TRY(builder.try_append("AnnounceCapabilities { "sv));
+    TRY(builder.try_appendff("is_request = {}, ", is_request()));
+    TRY(builder.try_appendff("capabilities.size() = {}", capabilities().size()));
+    TRY(builder.try_append(" }"sv));
+    return builder.to_string();
+}
+
+}