chore: keep the pluginWrapper bean for compatibility (#6271)

#### What type of PR is this?
/area core
/milestone 2.18.x

#### What this PR does / why we need it:
为了平滑升级先保留 PluginWrapper 的 Bean

#### Does this PR introduce a user-facing change?
```release-note
None
```

api/src/main/java/run/halo/app/plugin/BasePlugin.java

@@ -1,8 +1,8 @@
 package run.halo.app.plugin;
 
+import lombok.Getter;
 import lombok.extern.slf4j.Slf4j;
 import org.pf4j.Plugin;
-import org.springframework.lang.NonNull;
 
 /**
  * This class will be extended by all plugins and serve as the common class between a plugin and
@@ -11,10 +11,11 @@ import org.springframework.lang.NonNull;
  * @author guqing
  * @since 2.0.0
  */
+@Getter
 @Slf4j
 public class BasePlugin extends Plugin {
 
-    protected final PluginContext context;
+    protected PluginContext context;
 
     /**
      * Constructor a plugin with the given plugin context.
@@ -25,8 +26,6 @@ public class BasePlugin extends Plugin {
         this.context = pluginContext;
     }
 
-    @NonNull
-    public PluginContext getContext() {
-        return context;
+    public BasePlugin() {
     }
 }

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

@@ -85,6 +85,7 @@ public class DefaultPluginApplicationContextFactory implements PluginApplication
 
         sw.start("RegisterBeans");
         var beanFactory = context.getBeanFactory();
+        beanFactory.registerSingleton("pluginWrapper", pluginWrapper);
         context.registerBean(AggregatedRouterFunction.class);
 
         if (pluginWrapper.getPlugin() instanceof SpringPlugin springPlugin) {