Publish capabilities via MQTT (#254)

The idea is to have a map of capabilities that make it easy for apps to
check what the mower supports, without having to compare versions.

For simplicity in usage, the capabilities should be represented by a
simple, flat object with the respective capabilities as keys and the
"generation" as value. It starts with 1 when the capability gets added
and increases whenever a significant change was done.

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **New Features**
* System capabilities are now published as JSON on connection for
improved discovery.

* **Updates**
* Version publication now includes a JSON payload on an updated topic
path while retaining the existing binary publication.
* Capabilities publication occurs early during connection setup to
ensure consumers receive current state promptly.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Author: rovo89

src/lib/xbot_monitoring/src/capabilities.h

@@ -0,0 +1,7 @@
+#pragma once
+
+#include "nlohmann/json.hpp"
+
+inline const nlohmann::ordered_json CAPABILITIES = {
+    {"rpc", 1},
+};

src/lib/xbot_monitoring/src/xbot_monitoring.cpp

@@ -26,9 +26,11 @@
 #include "xbot_rpc/constants.h"
 #include "xbot_rpc/provider.h"
 #include "xbot_rpc/RegisterMethodsSrv.h"
+#include "capabilities.h"
 
 using json = nlohmann::ordered_json;
 
+void publish_capabilities();
 void publish_sensor_metadata();
 void publish_map();
 void publish_map_overlay();
@@ -74,6 +76,7 @@ class MqttCallback : public mqtt::callback {
 
     void connected(const mqtt::string &string) override {
         ROS_INFO_STREAM("MQTT Connected");
+        publish_capabilities();
         publish_sensor_metadata();
         publish_map();
         publish_map_overlay();
@@ -263,11 +266,15 @@ void publish_version() {
     json version = {
             {"version", version_string}
     };
-    try_publish("version", version.dump(), true);
+    try_publish("version/json", version.dump(), true);
     auto bson = json::to_bson(version);
     try_publish_binary("version", bson.data(), bson.size(), true);
 }
 
+void publish_capabilities() {
+  try_publish("capabilities/json", CAPABILITIES.dump(2), true);
+}
+
 void publish_sensor_metadata() {
     std::unique_lock<std::mutex> lk(mqtt_callback_mutex);