Hide the network class declaration to Network.cpp (#8276)

The Network class is not used by anyone other than free functions in
Network.cpp, nor should it be used by anyone with the current design.

This change leaves only the publicly-exposed functions left in the
header, drastically reducing amount of required headers in network.h,
one of the heaviest headers we had so far.

src/openrct2/network/NetworkConnection.cpp

@@ -14,6 +14,7 @@
 #    include "../core/String.hpp"
 #    include "../localisation/Localisation.h"
 #    include "../platform/platform.h"
+#    include "TcpSocket.h"
 #    include "network.h"
 
 constexpr size_t NETWORK_DISCONNECT_REASON_BUFFER_SIZE = 256;