Introduce PersistentProperty.isReadable #2915
Description
Based on spring-projects/spring-data-mongodb#4485, we should introduce a isReadable flag for easier downstream usage of truly immutable properties. The immutable flag just expresses immutability on the instance but requires checks for withers and Kotlin copy methods for creating a new instance. That isn't handy for values of Kotlin data classes (properties of a Kotlin data class that cannot be set via copy).