Add ValidationInterface to the DataObject class

[emteknetnz]

https://github.com/silverstripe/silverstripe-framework/blob/6/src/Core/Validation/ValidationInterface.php provides a standardised method signature for objects with a validate() method

So far this had been added to

• DBField
• FormField
• FieldValidator (via the subclass FieldValidationInterface)

At very least it should be added to DataObject which already effectively implements it, just with loose typing.

If feasible within a reasonable amount of it, it should also be added to as many other objects that currently have a validate($validator) or validate(): bool, which will involve some level of refactoring

Acceptance criteria

• ValidationInterface is added to DataObject, either directly or by getting DataObjectInterface to extend ValidationInterface
• Any other validate() methods that can be easily refactored also implement ValidationInterface