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();
```

Start with the CDK overlay ConnectedPositionStrategy API and reproduce the responsive-width case described in the issue. Done means a connected popup can match its parent width as the parent changes, without custom styling on the popup component; no specific source file or test is named in the report.

Written by the indexing model from the issue text.