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.

Start by locating the MatDatepicker and MatCalendar entry points and reviewing how the calendar instance is currently accessed through private members. Define a supported public access path that works when the datepicker is open and remains reliable with touch UI. Done means consumers can update the calendar state without relying on private members.

Written by the indexing model from the issue text.