[11.x] Update Date Validation to support custom date format

[mrvipchien]

This PR enhances the Date Validation logic by incorporating a configurable date format. Instead of relying on the default Y-m-d format, the Validator will now utilize the format specified in the format() method

[taylorotwell]

You can just format the date when you pass it into the before, after methods, etc. since they accept strings.