chore: provide post content retrieval bean for plugins (#5981)

#### What type of PR is this?
/kind improvement
/area core
/area plugin
/milestone 2.16.x

#### What this PR does / why we need it:
为插件提供文章内容获取的 bean 以简化文章内容获取

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

#### Does this PR introduce a user-facing change?
```release-note
为插件提供文章内容获取的 Bean
```

api/src/main/java/run/halo/app/content/PostContentService.java

@@ -0,0 +1,15 @@
+package run.halo.app.content;
+
+import reactor.core.publisher.Flux;
+import reactor.core.publisher.Mono;
+
+public interface PostContentService {
+
+    Mono<ContentWrapper> getHeadContent(String postName);
+
+    Mono<ContentWrapper> getReleaseContent(String postName);
+
+    Mono<ContentWrapper> getSpecifiedContent(String postName, String snapshotName);
+
+    Flux<String> listSnapshots(String postName);
+}

application/src/main/java/run/halo/app/content/PostContentServiceImpl.java

@@ -0,0 +1,58 @@
+package run.halo.app.content;
+
+import org.springframework.stereotype.Component;
+import reactor.core.publisher.Flux;
+import reactor.core.publisher.Mono;
+import run.halo.app.core.extension.content.Post;
+import run.halo.app.extension.ReactiveExtensionClient;
+import run.halo.app.extension.Ref;
+
+/**
+ * Provides ability to get post content for the specified post.
+ *
+ * @author guqing
+ * @since 2.16.0
+ */
+@Component
+public class PostContentServiceImpl extends AbstractContentService implements PostContentService {
+    private final ReactiveExtensionClient client;
+
+    public PostContentServiceImpl(ReactiveExtensionClient client) {
+        super(client);
+        this.client = client;
+    }
+
+    @Override
+    public Mono<ContentWrapper> getHeadContent(String postName) {
+        return client.get(Post.class, postName)
+            .flatMap(post -> {
+                var headSnapshot = post.getSpec().getHeadSnapshot();
+                return super.getContent(headSnapshot, post.getSpec().getBaseSnapshot());
+            });
+    }
+
+    @Override
+    public Mono<ContentWrapper> getReleaseContent(String postName) {
+        return client.get(Post.class, postName)
+            .flatMap(post -> {
+                var releaseSnapshot = post.getSpec().getReleaseSnapshot();
+                return super.getContent(releaseSnapshot, post.getSpec().getBaseSnapshot());
+            });
+    }
+
+    @Override
+    public Mono<ContentWrapper> getSpecifiedContent(String postName, String snapshotName) {
+        return client.get(Post.class, postName)
+            .flatMap(post -> {
+                var baseSnapshot = post.getSpec().getBaseSnapshot();
+                return super.getContent(snapshotName, baseSnapshot);
+            });
+    }
+
+    @Override
+    public Flux<String> listSnapshots(String postName) {
+        return client.get(Post.class, postName)
+            .flatMapMany(page -> listSnapshotsBy(Ref.of(page)))
+            .map(snapshot -> snapshot.getMetadata().getName());
+    }
+}

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

@@ -3,6 +3,7 @@ package run.halo.app.plugin;
 import org.springframework.context.ApplicationContext;
 import org.springframework.context.support.GenericApplicationContext;
 import org.springframework.security.web.server.context.ServerSecurityContextRepository;
+import run.halo.app.content.PostContentService;
 import run.halo.app.core.extension.service.AttachmentService;
 import run.halo.app.extension.DefaultSchemeManager;
 import run.halo.app.extension.ExtensionClient;
@@ -53,6 +54,8 @@ public enum SharedApplicationContextFactory {
             rootContext.getBean(NotificationCenter.class));
         beanFactory.registerSingleton("externalLinkProcessor",
             rootContext.getBean(ExternalLinkProcessor.class));
+        beanFactory.registerSingleton("postContentService",
+            rootContext.getBean(PostContentService.class));
         // TODO add more shared instance here
 
         sharedContext.refresh();