CUB-230: ESP32 offline status buffering and replay on reconnect

- Add SPIFFS buffer for status entries when MQTT offline
- FIFO eviction when max_buffer_entries exceeded
- Replay buffer on MQTT reconnect
- Configurable buffer size in esp32-config.json

firmware/.pio/libdeps/esp32-mqtt/PubSubClient/examples/mqtt_auth/mqtt_auth.ino

@@ -0,0 +1,43 @@
+/*
+ Basic MQTT example with Authentication
+
+  - connects to an MQTT server, providing username
+    and password
+  - publishes "hello world" to the topic "outTopic"
+  - subscribes to the topic "inTopic"
+*/
+
+#include <SPI.h>
+#include <Ethernet.h>
+#include <PubSubClient.h>
+
+// Update these with values suitable for your network.
+byte mac[]    = {  0xDE, 0xED, 0xBA, 0xFE, 0xFE, 0xED };
+IPAddress ip(172, 16, 0, 100);
+IPAddress server(172, 16, 0, 2);
+
+void callback(char* topic, byte* payload, unsigned int length) {
+  // handle message arrived
+}
+
+EthernetClient ethClient;
+PubSubClient client(server, 1883, callback, ethClient);
+
+void setup()
+{
+  Ethernet.begin(mac, ip);
+  // Note - the default maximum packet size is 128 bytes. If the
+  // combined length of clientId, username and password exceed this use the
+  // following to increase the buffer size:
+  // client.setBufferSize(255);
+  
+  if (client.connect("arduinoClient", "testuser", "testpass")) {
+    client.publish("outTopic","hello world");
+    client.subscribe("inTopic");
+  }
+}
+
+void loop()
+{
+  client.loop();
+}