feat(MatDatepicker): Expose access to MatCalendar used in MatDatepicker

#### Feature Description

Expose access to MatCalendar instance that is created under the hood when using datepicker.

#### Use Case

I have requirement to allow user to type into the input date in proper format, and if datepicker is opened, it should update its state by showing typed in date. This is not possible using current api state. What I do now, is a nasty hacking - accessing private members of datepicker.

so, in my component, I have to
```

get calendar(): MatCalendar | undefined {
// this is soooo baaad and soo hacky to get that calendar instance......
// moreover, calendar can actually be null - when its closed.
// @ts-ignore
return this.datepicker._popupComponentRef?.instance._calendar;
}

this.myInputField.valueChanges.pipe(filter(v => v), debounceTime(250)).subscribe(v => {
const calendar = this.calendar;
// console.log('dp', this.datepicker);
// console.log('cal', this.calendar);
//sooooooooo much haaaaaaaaaaaxing
if (calendar) {
calendar.activeDate = v;
}
```

I think its perfectly valid to manipulate calendar state from the component code. This will obviously stop working when I swtich to touchUI. Therfore, API that provides reliable way to access calendar would be even more convinient.

まず MatDatepicker と MatCalendar のエントリポイントを特定し、現在カレンダーインスタンスに private メンバー経由でアクセスしている方法を確認します。datepicker が開いているときに機能し、タッチ UI でも信頼性を維持できる、サポート対象の public なアクセス経路を定義します。consumer が private メンバーに依存せずにカレンダーの状態を更新できれば完了です。

索引モデルが issue の本文から書いたものです。