chore: add issues field for plugin manifest file (#30)

为插件定义文件添加 issues 字段。

Refer halo-dev/halo#5755

```release-note
None
```

Author: ruibaby

src/main/resources/plugin.yaml

@@ -14,6 +14,7 @@ spec:
   logo: logo.png
   homepage: https://github.com/halo-dev/plugin-starter#readme
   repo: https://github.com/halo-dev/plugin-starter
+  issues: https://github.com/halo-dev/plugin-starter/issues
   displayName: "插件快速开始模板"
   description: "这是一个插件快速开始模板"
   license: