CDK overlay auto calculate width when using ConnectedPositionStrategy

#### Bug, feature request, or proposal:
Feature

#### What is the expected behavior?
That i can somehow configure the popup overlay to change it width to be the same as the parent, without having to add a bunch of custom styling to my popup component.

#### What is the current behavior?

#### What is the use-case or motivation for changing an existing behavior?
I would have to do less custom styling to get a nice dropdown

#### Is there anything else we should know?
In the screenshots we have changed the width using css, however this is suboptimal to having a responsive design, where the input can change their width all the time.
To get around the problem we have to hard-code the width of both the input and the popups, so they look to fit together.

My best suggestion would be able to do something like this:

```typescript
this.overlay.position()
.connectedTo(target, {
originY: 'bottom',
originX: 'start'
}, {
overlayY: 'top',
overlayX: 'start'
})
.immitateWidth();
```

从 CDK overlay ConnectedPositionStrategy API 开始，重现 issue 中描述的响应式宽度场景。完成标准是：连接的弹出窗口能够在父元素发生变化时匹配其宽度，而无需对弹出窗口组件进行自定义样式设置；报告中没有指定具体的源文件或测试。

由索引模型根据 Issue 内容生成。