Move validate methods to new class-validate.php

[mauteri]

Currently there are a bunch validate methods in class-event-rest-api.php that I'd like to move to a general validate class.

We can remove the validate_ prefix from these methods when we move them. Make them static methods, eg, validate_event_post_id method will be Validate::event_post_id.

Unit tests will need to be moved as well.

[JordanPak]

I'ma take a shot at this one :)