feat(datepicker): Improve extensibility for datepicker use custom calendar implementation

[martin-the-geek]

Feature Description

The content of the datepicker overlay is hard-coded in:

components/src/material/datepicker/datepicker-base.ts

Lines 685 to 688 in 36af2a3

	const portal = new ComponentPortal<MatDatepickerContent<S, D>>(
	MatDatepickerContent,
	this._viewContainerRef,
	);

It would be beneficial to allow custom content implementations (either derived from MatDatepickerContent or matching an interface) to be used, maybe as a factory method that could be added as a provider.

Use Case

We have an application where weeks are grouped into biweekly periods and the users find it useful when selecting dates which biweekly period a week is in and whether the week is the first or second week.

We are able to subtype the MatCalendarBody with a custom template that include another column (with a rowspan) to identify the biweekly period.

We are able to subtype the MatMonthView to use our custom body and to ensure the cells are built with days in the months before or after to make sure that each biweekly period is complete (it would be nice if _createWeekCells, _getCellCompareValue and _shouldEnableDate were not private so that they could be overridden/accessed).

We are able to subtype MatCalendar to use our custom month view.

We are able to subtype MatDatepickerContent to use our custom calendar.

Unfortunately this is where it ends. We cannot use a custom content component with the datepicker without copying a large amount of code.

At a minimum we would like to be able to specify a custom content for the datepicker (and dateRangePicker) through something like a provider.

Or even nicer would be an option to dynamically identify the calendar views (possibly with a provider) to avoid all the extra subtyping.

This extensibility could also help other achieve similar requests like #20530.