core/network: Add network abstraction

This commit adds a network abstraction designed to implement bsd:s but
at the same time work as a generic abstraction to implement any
networking code we have to use from core.

This is implemented on top of BSD sockets on Unix systems and winsock on
Windows. The code is designed around winsocks having compatibility
definitions to support both BSD and Windows sockets.

src/core/core.cpp

@@ -43,6 +43,7 @@
 #include "core/loader/loader.h"
 #include "core/memory.h"
 #include "core/memory/cheat_engine.h"
+#include "core/network/network.h"
 #include "core/perf_stats.h"
 #include "core/reporter.h"
 #include "core/settings.h"
@@ -394,6 +395,9 @@ struct System::Impl {
     /// Telemetry session for this emulation session
     std::unique_ptr<Core::TelemetrySession> telemetry_session;
 
+    /// Network instance
+    Network::NetworkInstance network_instance;
+
     ResultStatus status = ResultStatus::Success;
     std::string status_details = "";