values: Include boot count in state messages

This should be useful for diagnostic purposes.  It's not brought out
into a proper sensor in HA, but it can be seen using the MQTT message
viewer in the HA device configuration tool.

gardensensor.ino

@@ -52,6 +52,7 @@ void setup() {
         reboot();
     }
 
+    values.boot_count = boot_count;
     if (!values.read(&ss)) {
         auto msg = "Failed to get sensor values";
         Serial.println(msg);
@@ -63,6 +64,7 @@ void setup() {
         Serial.printf("  Temperature: %.02f\n", values.temperature);
         Serial.printf("  Battery: %.02f\n", values.battery);
         Serial.printf("  RSSI: %d\n", values.rssi);
+        Serial.printf("  Boot Count: %d\n", values.boot_count);
         if (!values.send(&mqtt, TOPIC_STATE)) {
             Serial.println("Failed to send sensor values");
         }

values.cpp

@@ -16,11 +16,12 @@ bool Values::read(Adafruit_seesaw* ss) {
 }
 
 bool Values::send(PubSubClient* mqtt, const char* topic) {
-    StaticJsonDocument<64> doc;
+    StaticJsonDocument<96> doc;
     doc["moisture"] = moisture;
     doc["temperature"] = round2(temperature);
     doc["battery_level"] = round2(battery);
     doc["rssi"] = rssi;
+    doc["boot_count"] = boot_count;
 
     String msg;
     serializeJson(doc, msg);

values.hpp

@@ -7,6 +7,7 @@ class Values {
     float temperature;
     float battery;
     int8_t rssi;
+    uint32_t boot_count;
 
     Values() {};
     ~Values() {};