Implement Unavailable Dates for Date Picker

[dgodinez-dh]

For #367, the initial implementation has the unavailable_dates prop commented out.

This list of dates was sent over to the Javascript which create an isDateUnavailable function by checking if the date provided by the Spectrum API was included in the Set of dates proved by Python. The problem is that these dates have granularity down to the millisecond which means that any small change in the time will cause the comparison to fail. The Javascript needs to be smarter. It should drop any time information and just do comparison on the CalendarDate.

We might need to rethink the Python API. Is there any point saying 2024-01-01T12:10:30 is unavailable vs just 2024-01-01