feat: add site stats finder for theme-side (#2604)

#### What type of PR is this?
/kind feature
/area core
/milestone 2.0

#### What this PR does / why we need it:
提供主题端站点统计信息查询器
#### Which issue(s) this PR fixes:

Fixes #

#### Special notes for your reviewer:
how to test it?
在任意主题页面使用如下语法
```
<p th:text="${siteStatsFinder.getStats()}"></p>
```
/cc @halo-dev/sig-halo 
#### Does this PR introduce a user-facing change?

```release-note
提供主题端站点统计信息查询器
```

src/main/java/run/halo/app/theme/finders/SiteStatsFinder.java

@@ -0,0 +1,14 @@
+package run.halo.app.theme.finders;
+
+import run.halo.app.theme.finders.vo.SiteStatsVo;
+
+/**
+ * Site statistics finder.
+ *
+ * @author guqing
+ * @since 2.0.0
+ */
+public interface SiteStatsFinder {
+
+    SiteStatsVo getStats();
+}

src/main/java/run/halo/app/theme/finders/impl/SiteStatsFinderImpl.java

@@ -0,0 +1,55 @@
+package run.halo.app.theme.finders.impl;
+
+import lombok.AllArgsConstructor;
+import reactor.core.publisher.Mono;
+import run.halo.app.core.extension.Category;
+import run.halo.app.core.extension.Counter;
+import run.halo.app.core.extension.Post;
+import run.halo.app.extension.ReactiveExtensionClient;
+import run.halo.app.theme.finders.Finder;
+import run.halo.app.theme.finders.SiteStatsFinder;
+import run.halo.app.theme.finders.vo.SiteStatsVo;
+
+/**
+ * A default implementation of {@link SiteStatsFinder}.
+ *
+ * @author guqing
+ * @since 2.0.0
+ */
+@AllArgsConstructor
+@Finder("siteStatsFinder")
+public class SiteStatsFinderImpl implements SiteStatsFinder {
+    private final ReactiveExtensionClient client;
+
+    @Override
+    public SiteStatsVo getStats() {
+        return client.list(Counter.class, null, null)
+            .reduce(SiteStatsVo.empty(), (stats, counter) -> {
+                stats.setVisit(stats.getVisit() + counter.getVisit());
+                stats.setComment(stats.getComment() + counter.getApprovedComment());
+                stats.setUpvote(stats.getUpvote() + counter.getUpvote());
+                return stats;
+            })
+            .flatMap(siteStatsVo -> postCount()
+                .doOnNext(siteStatsVo::setPost)
+                .thenReturn(siteStatsVo)
+            )
+            .flatMap(siteStatsVo -> categoryCount()
+                .doOnNext(siteStatsVo::setCategory)
+                .thenReturn(siteStatsVo))
+            .block();
+    }
+
+    Mono<Integer> postCount() {
+        return client.list(Post.class, post -> !post.isDeleted() && post.isPublished(), null)
+            .count()
+            .map(Long::intValue);
+    }
+
+    Mono<Integer> categoryCount() {
+        return client.list(Category.class, null, null)
+            .count()
+            .map(Long::intValue);
+    }
+
+}

src/main/java/run/halo/app/theme/finders/vo/SiteStatsVo.java

@@ -0,0 +1,35 @@
+package run.halo.app.theme.finders.vo;
+
+import lombok.Builder;
+import lombok.Data;
+
+/**
+ * A value object for site stats.
+ *
+ * @author guqing
+ * @since 2.0.0
+ */
+@Data
+@Builder
+public class SiteStatsVo {
+
+    private Integer visit;
+
+    private Integer upvote;
+
+    private Integer comment;
+
+    private Integer post;
+
+    private Integer category;
+
+    public static SiteStatsVo empty() {
+        return SiteStatsVo.builder()
+            .visit(0)
+            .upvote(0)
+            .comment(0)
+            .post(0)
+            .category(0)
+            .build();
+    }
+}