Add CaptureMessage

This commit adds the CaptureMessage function.  This will later be called
when any message is sent or received.  The capture directory is fixed,
in a new folder "message_capture" in the datadir.  Peers will then have
their own subfolders, named with their IP address and port, replacing
colons with underscores to keep compatibility with Windows.  Inside,
received and sent messages will be captured into two binary files,
msgs_recv.dat and msgs_sent.dat.

e.g.
message_capture/203.0.113.7_56072/msgs_recv.dat
message_capture/203.0.113.7_56072/msgs_sent.dat

The format has been designed as to result in a minimal performance
impact.  A parsing script is added in a later commit.

src/net.cpp

@@ -2929,3 +2929,31 @@ uint64_t CConnman::CalculateKeyedNetGroup(const CAddress& ad) const
 
     return GetDeterministicRandomizer(RANDOMIZER_ID_NETGROUP).Write(vchNetGroup.data(), vchNetGroup.size()).Finalize();
 }
+
+void CaptureMessage(const CAddress& addr, const std::string& msg_type, const Span<const unsigned char>& data, bool is_incoming)
+{
+    // Note: This function captures the message at the time of processing,
+    // not at socket receive/send time.
+    // This ensures that the messages are always in order from an application
+    // layer (processing) perspective.
+    auto now = GetTime<std::chrono::microseconds>();
+
+    // Windows folder names can not include a colon
+    std::string clean_addr = addr.ToString();
+    std::replace(clean_addr.begin(), clean_addr.end(), ':', '_');
+
+    fs::path base_path = GetDataDir() / "message_capture" / clean_addr;
+    fs::create_directories(base_path);
+
+    fs::path path = base_path / (is_incoming ? "msgs_recv.dat" : "msgs_sent.dat");
+    CAutoFile f(fsbridge::fopen(path, "ab"), SER_DISK, CLIENT_VERSION);
+
+    ser_writedata64(f, now.count());
+    f.write(msg_type.data(), msg_type.length());
+    for (auto i = msg_type.length(); i < CMessageHeader::COMMAND_SIZE; ++i) {
+        f << '\0';
+    }
+    uint32_t size = data.size();
+    ser_writedata32(f, size);
+    f.write((const char*)data.data(), data.size());
+}