Throw an exception in ConfigEntity::save() (or even Entity::save() ?) if the UUID is changed

Followup from #1969698: Field UUID changes can badly corrupt field data, postponed on discussion in #1969800: [META] Discuss how and whether the configuration system, config entities, or their implementations should or shouldn't use UUIDs.

We're adding this exception handling to both Field and FieldInstance. Should ConfigEntity (or even entity) enforce that the UUID cannot change for an existing entity?