Extending CdkTextareaAutosize features. heightChanges / setHeight

## Proposal

Add for [MatTextareaAutosize](https://material.angular.io/components/input/api#MatTextareaAutosize) Callback with height and return custom sum and set it as height for textarea element.

```
CdkTextareaAutosize {
...
// Options by default parameter emitEvent is false
setHeight(height: number, {emitEvent: boolean}): void;

// Return number after calculation height textarea element
heightChanges(): Observable;
...
}
```

Start by reading the existing MatTextareaAutosize/CdkTextareaAutosize API and implementation, using the linked Angular Material API reference as context. Determine how the proposed setHeight and heightChanges behavior should fit the current component, and consider the proposal complete when both methods have clearly defined, working behavior for textarea height updates and event emission.

Written by the indexing model from the issue text.