Expose ReactiveUserDetailsService to authentication plugins (#6885)

#### What type of PR is this?

/kind improvement
/area core
/milestone 2.20.x

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

This PR exposes ReactiveUserDetailsService to authentication plugins.

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

```release-note
支持在插件中使用 ReactiveUserDetailsService
```

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

@@ -4,6 +4,7 @@
 import org.springframework.cache.CacheManager;
 import org.springframework.context.ApplicationContext;
 import org.springframework.context.support.GenericApplicationContext;
+import org.springframework.security.core.userdetails.ReactiveUserDetailsService;
 import org.springframework.security.web.server.context.ServerSecurityContextRepository;
 import org.springframework.security.web.server.savedrequest.ServerRequestCache;
 import run.halo.app.content.PostContentService;
@@ -92,6 +93,10 @@ public static ApplicationContext create(ApplicationContext rootContext) {
             .ifUnique(userService -> beanFactory.registerSingleton("userService", userService));
         rootContext.getBeanProvider(RoleService.class)
             .ifUnique(roleService -> beanFactory.registerSingleton("roleService", roleService));
+        rootContext.getBeanProvider(ReactiveUserDetailsService.class)
+            .ifUnique(userDetailsService ->
+                beanFactory.registerSingleton("userDetailsService", userDetailsService)
+            );
         // TODO add more shared instance here
 
         sharedContext.refresh();