feat: add database endpoint for actuator (#3351)

#### What type of PR is this?
/kind feature

/milestone 2.3.x

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

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

Fixes #3263

#### Special notes for your reviewer:
测试方法：
1. 使用超级管理员登录
2. 查看接口 /actuator/info 查看是否存在 database 字段。
#### Does this PR introduce a user-facing change?


```release-note
NONE
```

src/main/java/run/halo/app/actuator/DatabaseInfoContributor.java

@@ -0,0 +1,48 @@
+package run.halo.app.actuator;
+
+import io.r2dbc.spi.Connection;
+import io.r2dbc.spi.ConnectionFactory;
+import io.r2dbc.spi.ConnectionMetadata;
+import java.util.HashMap;
+import java.util.Map;
+import java.util.Objects;
+import org.springframework.boot.actuate.info.Info;
+import org.springframework.boot.actuate.info.InfoContributor;
+import org.springframework.stereotype.Component;
+import reactor.core.publisher.Mono;
+
+@Component
+public class DatabaseInfoContributor implements InfoContributor {
+    private static final String DATABASE_INFO_KEY = "database";
+
+    private final ConnectionFactory connectionFactory;
+
+    public DatabaseInfoContributor(ConnectionFactory connectionFactory) {
+        this.connectionFactory = connectionFactory;
+    }
+
+    @Override
+    public void contribute(Info.Builder builder) {
+        builder.withDetail(DATABASE_INFO_KEY, contributorMap());
+    }
+
+    public Map<String, Object> contributorMap() {
+        var map = new HashMap<String, Object>();
+        var connectionMetadata = getConnectionMetadata().block();
+        if (Objects.isNull(connectionMetadata)) {
+            return map;
+        }
+        map.put("name", connectionMetadata.getDatabaseProductName());
+        map.put("version", connectionMetadata.getDatabaseVersion());
+        return map;
+    }
+
+    private Mono<ConnectionMetadata> getConnectionMetadata() {
+        return Mono.usingWhen(this.connectionFactory.create(),
+            conn -> Mono.just(conn.getMetadata()),
+            Connection::close,
+            (conn, t) -> conn.close(),
+            Connection::close
+        );
+    }
+}