button icon 支持表达式

#### 是否关联于某个问题吗：
目前 button 的 icon 无法通过表达式，动态获取，希望能够通过表达式动态获取

#### 预期的解决方案：
希望以下表达式能够正常渲染出 icon

```json
{
"type": "page",
"data": {
"url": "https://suda.cdn.bcebos.com/images%2F2021-01%2Fdiamond.svg",
"name": "测试"
},
"body": {
"label": "${name}",
"type": "button",
"actionType": "dialog",
"icon": "${url}",
"dialog": {
"title": "弹框",
"body": "这是个简单的弹框。"
}
}
}
```

#### 其他可接受方案：
无

#### 任何附加信息：
无

Start by locating the button component's icon rendering and the existing expression-evaluation path. Use the provided page configuration to check whether a data value can resolve the button's icon URL, and consider the issue done when that example renders the diamond icon correctly.

Written by the indexing model from the issue text.