Add a choice to use UUID in entity reference values instead of the entity's ID

Instead of using an entity's primary ID, entity references should use the entity's UUID.
But the change is so huge for Drupal8,
So we can add a choice to use UUID in entity reference values instead of the entity's ID when create a new entity reference field.

It mill make help for

1. remote entity reference
2. migrations, e.g. #2748609: [meta] Preserving auto-increment IDs on migration is fragile.

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