Allow plugin to listen the event the plugin has started (#6234)

#### What type of PR is this?

/kind feature
/area core
/milestone 2.17.x

#### What this PR does / why we need it:

This PR add support for allowing plugin to listen the event that the plugin has started. Below is an example of listening the event in plugin:

```java
    @eventlistener
    void onPluginStartedEvent(PluginStartedEvent event) {
        // do something.
    }
```

See #5339 (comment) for more.

#### Which issue(s) this PR fixes:

Fixes #5339 (comment)

#### Special notes for your reviewer:

1. Create a plugin, add the listener above and write some logs
2. Build and install the plugin
3. Start plugin and see the logs you wrote

#### Does this PR introduce a user-facing change?

```release-note
支持在插件中监听已启动事件
```

api/src/main/java/run/halo/app/plugin/event/PluginStartedEvent.java

@@ -0,0 +1,17 @@
+package run.halo.app.plugin.event;
+
+import org.springframework.context.ApplicationEvent;
+
+/**
+ * The event that is published when a plugin is really started, and is only for plugin internal use.
+ *
+ * @author johnniang
+ * @since 2.17.0
+ */
+public class PluginStartedEvent extends ApplicationEvent {
+
+    public PluginStartedEvent(Object source) {
+        super(source);
+    }
+
+}

application/src/main/java/run/halo/app/plugin/HaloPluginManager.java

@@ -20,11 +20,15 @@
 import org.pf4j.PluginFactory;
 import org.pf4j.PluginLoader;
 import org.pf4j.PluginRepository;
+import org.pf4j.PluginState;
+import org.pf4j.PluginStateEvent;
+import org.pf4j.PluginStateListener;
 import org.pf4j.PluginStatusProvider;
 import org.pf4j.PluginWrapper;
 import org.springframework.context.ApplicationContext;
 import org.springframework.data.util.Lazy;
 import run.halo.app.infra.SystemVersionSupplier;
+import run.halo.app.plugin.event.PluginStartedEvent;
 
 /**
  * PluginManager to hold the main ApplicationContext.
@@ -56,6 +60,9 @@ public HaloPluginManager(ApplicationContext rootContext,
         setSystemVersion(systemVersionSupplier.get().getNormalVersion());
 
         super.initialize();
+
+        // the listener must be after the super#initialize
+        addPluginStateListener(new PluginStartedListener());
     }
 
     @Override
@@ -175,4 +182,30 @@ public List<PluginWrapper> getDependents(String pluginId) {
         }
         return dependents;
     }
+
+    /**
+     * Listener for plugin started event.
+     *
+     * @author johnniang
+     * @since 2.17.0
+     */
+    private static class PluginStartedListener implements PluginStateListener {
+
+        @Override
+        public void pluginStateChanged(PluginStateEvent event) {
+            if (PluginState.STARTED.equals(event.getPluginState())) {
+                var plugin = event.getPlugin().getPlugin();
+                if (plugin instanceof SpringPlugin springPlugin) {
+                    try {
+                        springPlugin.getApplicationContext()
+                            .publishEvent(new PluginStartedEvent(this));
+                    } catch (Throwable t) {
+                        var pluginId = event.getPlugin().getPluginId();
+                        log.warn("Error while publishing plugin started event for plugin {}",
+                            pluginId, t);
+                    }
+                }
+            }
+        }
+    }
 }