Watermark水印组件的字体无法设置opacity半透明样式

- [ ] I have searched the [issues](https://github.com/vueComponent/ant-design-vue/issues)         of this repository and believe that this is not a duplicate.
        
        
### What problem does this feature solve?
Font属性上面可以设置字体opacity透明样式

### What does the proposed API look like?
支持字体opacity属性