Bug 33029: Add wrapper method for dt_from_string to KohaDates template toolkit plugin

Bug 33029: Add wrapper method for dt_from_string to KohaDates template toolkit plugin

It would be very useful to have direct access to dt_from_string in our templates. This would allow for us to handle custom date and time formatting. It would, for example, allow us to output the month name for a given date via Template Toolkit easily.

Test Plan:
1) Apply this patch
2) In a notice add '[% Use KohaDates %][% KohaDates.datetime_from_string().ymd %]' to the top of a notice
3) Generate that notice for a patron
4) Note today's date in iso format is rendered at the top of the notice

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Aleisha Amohia <aleishaamohia@hotmail.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>