feat(mqtt): add rflab.io WebSocket broker preset

Adds the "rflab" built-in MQTT preset and bumps MQTT_PRESET_COUNT to 22.

  - Server:   wss://mqtt.rflab.io:443
  - Auth:     JWT (Ed25519 device identity), audience mqtt.rflab.io
  - Topic:    MeshCore style
  - TLS:      Let's Encrypt, pinned to ISRG Root X1

Also documents the preset in the MQTT_IMPLEMENTATION.md preset table.

src/helpers/MQTTPresets.h

@@ -105,7 +105,7 @@ static const char ISRG_ROOT_X1[] PROGMEM =
     "-----END CERTIFICATE-----\n";
 
 // Number of built-in presets
-static const int MQTT_PRESET_COUNT = 21;
+static const int MQTT_PRESET_COUNT = 22;
 
 // Built-in preset definitions (stored in flash)
 static const MQTTPresetDef MQTT_PRESETS[MQTT_PRESET_COUNT] = {
@@ -131,6 +131,7 @@ static const MQTTPresetDef MQTT_PRESETS[MQTT_PRESET_COUNT] = {
     { "meshcore-ca-2",   "wss://mqtt2.meshcore.ca:443/mqtt",          "mqtt2.meshcore.ca",               ISRG_ROOT_X1, MQTT_AUTH_JWT,      MQTT_TOPIC_MESHCORE,  0,       true,   55,      nullptr,     nullptr     },
     { "inwmesh",         "mqtts://scope.inwmesh.org:8883",            nullptr,                           ISRG_ROOT_X1,  MQTT_AUTH_USERPASS,  MQTT_TOPIC_MESHCORE,  0,       true,   55,      nullptr,     nullptr     },
     { "bostonmesh",      "wss://mqttmc01.bostonme.sh:443/mqtt",       "mqttmc01.bostonme.sh",            GTS_ROOT_R4,   MQTT_AUTH_JWT,      MQTT_TOPIC_MESHCORE,  0,       true,   55,      nullptr,     nullptr     },
+    { "rflab",           "wss://mqtt.rflab.io:443",                   "mqtt.rflab.io",                   ISRG_ROOT_X1,  MQTT_AUTH_JWT,      MQTT_TOPIC_MESHCORE,  0,       true,   55,      nullptr,     nullptr     },
 };
 
 // Find a preset by name, returns nullptr if not found