new usermod hooks "onUdpPacket"

this new hooks will help you implement new and custom protocols in
usermods.
I've provided an example (see usermods/udp_name_sync).
The example will help you share the main segment name across different
WLED instances.
The segment name can be useful to sync with some effects like GIF
image or scrolling text.

If you define new packet format in your usermod, make sure it will
either not collide with already used version of wled udp packet :
- 0 is for udp sync
- 1 is for AudioReactive data
- 2 is for udp_name_sync :)

Also, the onUdpPacket will override "parseNotification" if it returns "true".
Have fun!

usermods/udp_name_sync/library.json

@@ -0,0 +1,5 @@
+{
+  "name": "udp_name_sync",
+  "build": { "libArchive": false },
+  "dependencies": {}
+}

usermods/udp_name_sync/udp_name_sync.cpp

@@ -0,0 +1,79 @@
+#include "wled.h"
+
+class UdpNameSync : public Usermod {
+
+  private:
+
+    bool enabled = false;
+    bool initDone = false;
+    unsigned long lastTime = 0;
+    char segmentName[WLED_MAX_SEGNAME_LEN] = {0};
+    static const char _name[];
+    static const char _enabled[];
+
+  public:
+    /**
+     * Enable/Disable the usermod
+     */
+    inline void enable(bool enable) { enabled = enable; }
+
+    /**
+     * Get usermod enabled/disabled state
+     */
+    inline bool isEnabled() { return enabled; }
+
+    void setup() override {
+      initDone = true;
+    }
+
+    void loop() override {
+      if (!WLED_CONNECTED) return;
+      if (!udpConnected) return;
+      Segment& mainseg = strip.getMainSegment();
+      if (!strlen(segmentName) && !mainseg.name) return; //name was never set, do nothing
+
+      IPAddress broadcastIp = ~uint32_t(Network.subnetMask()) | uint32_t(Network.gatewayIP());
+      byte udpOut[WLED_MAX_SEGNAME_LEN + 2];
+      udpOut[0] = 2; // 0: wled notifier protocol, 1: warls protocol, 2 is free
+      
+      if (strlen(segmentName) && !mainseg.name) { //name is back to null
+	notifierUdp.beginPacket(broadcastIp, udpPort);
+	strcpy(segmentName,"");
+	DEBUG_PRINTLN(F("UdpNameSync: sending Null name"));
+	notifierUdp.write( udpOut , 2);
+	notifierUdp.endPacket();
+	return;
+      }
+
+      if (0 == strcmp(mainseg.name, segmentName)) return; //same name, do nothing
+
+      notifierUdp.beginPacket(broadcastIp, udpPort);
+      DEBUG_PRINT(F("UdpNameSync: saving segment name "));
+      DEBUG_PRINTLN(mainseg.name);
+      byte length = strlen(mainseg.name);
+      strlcpy(segmentName, mainseg.name, length+1);
+      strlcpy((char *)&udpOut[1], segmentName, length+1);
+      notifierUdp.write(udpOut, length + 2);
+      notifierUdp.endPacket();
+      DEBUG_PRINT(F("UdpNameSync: Sent segment name : "));
+      DEBUG_PRINTLN(segmentName);
+    }
+
+    bool onUdpPacket(uint8_t * payload, uint8_t len) override {
+      DEBUG_PRINT(F("UdpNameSync: Received packet"));
+      if (payload[0] != 2) return false;
+      //else
+      Segment& mainseg = strip.getMainSegment();
+      mainseg.setName((char *)&payload[1]);
+      DEBUG_PRINT(F("UdpNameSync: set segment name"));
+      return true;
+    }
+};
+
+
+// add more strings here to reduce flash memory usage
+const char UdpNameSync::_name[]    PROGMEM = "UdpNameSync";
+const char UdpNameSync::_enabled[] PROGMEM = "enabled";
+
+static UdpNameSync udp_name_sync;
+REGISTER_USERMOD(udp_name_sync);