Add validation for date parameters

Problem/Motivation

Dates are being input into "special" format, but there is no validation for the format. This would be needed for better user xp.

It could be added as trait and used on these three occasions:

https://git.drupalcode.org/project/ohdear_integration/-/blob/2.x/src/OhD...

https://git.drupalcode.org/project/ohdear_integration/-/blob/2.x/src/Dru...

https://git.drupalcode.org/project/ohdear_integration/-/blame/2.x/src/Co...