`input_date()` should allow strings as input

[wch]

Currently input_date() takes date objects as input. This is a bit inconvenient if the user just wants to provide the date as an ISO-formatted string. They currently have to do something like this:

from datetime import date

input_date("x", "x", date.fromisoformat("2022-01-01"))

It would be nice to be able to do it this way:

input_date("x", "x", "2022-01-01")

And same for input_date_range(), and possibly input_slider().