[DX] [D8] Add a `validate()` method to entity classes

[avpaderno]

In Drupal 8, classes that are used in order to implement a content entity (which extend the ContentEntityBase class) have a validate() method that validates the values entered when creating the entity.

A similar method should be introduced in Backdrop and added to EntityInterface.

[avpaderno]

This is probably a Backdrop 2.x feature. Contributed modules could have implemented a validate() method in their entities that could be not compatible with the implementation Backdrop adds.

[argiepiano]

This is probably a Backdrop 2.x feature.

Agreed. This would be a fairly complex and deep change to the way Backdrop deals with entities and data in general (e.g. fields). It's not just about creating a validate method for the entity class. The Drupal ContentEntityBase::validate() method actually calls the validate method for the entity's "typed data" objects - for example, its fields and properties.