Add CSerializedNetMsg::Copy() helper

This makes code that uses the helper less verbose.

Moreover, this makes net_processing C++20 compliant. Otherwise, it would
lead to a compile error (see below). C++20 disables aggregate
initialization when any constructor is declared. See
http://open-std.org/JTC1/SC22/WG21/docs/papers/2018/p1008r1.pdf

net_processing.cpp:1627:42: error: no matching constructor for initialization of 'CSerializedNetMsg'
            m_connman.PushMessage(pnode, CSerializedNetMsg{ser_cmpctblock.data, ser_cmpctblock.m_type});
                                         ^                ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

src/net.h

@@ -99,15 +99,22 @@ struct AddedNodeInfo
 class CNodeStats;
 class CClientUIInterface;
 
-struct CSerializedNetMsg
-{
+struct CSerializedNetMsg {
     CSerializedNetMsg() = default;
     CSerializedNetMsg(CSerializedNetMsg&&) = default;
     CSerializedNetMsg& operator=(CSerializedNetMsg&&) = default;
-    // No copying, only moves.
+    // No implicit copying, only moves.
     CSerializedNetMsg(const CSerializedNetMsg& msg) = delete;
     CSerializedNetMsg& operator=(const CSerializedNetMsg&) = delete;
 
+    CSerializedNetMsg Copy() const
+    {
+        CSerializedNetMsg copy;
+        copy.data = data;
+        copy.m_type = m_type;
+        return copy;
+    }
+
     std::vector<unsigned char> data;
     std::string m_type;
 };