支持自定义字段,类型为 `relation`

**Is your feature request related to a problem? Please describe.** Whenever I define a relation on an entity, a foreign key is created (duh), but I do not have the ability to specify what happens to the column (or row) when the referenced column is updated or deleted. Let's say I have a plugin that has a custom field definition on the `Product` entity that relates to a custom entity: