Mutable objects used for immutable values

### Describe the bug

`PolarisEntityCore` type hierarchy is used to represent the persisted (or about to be persisted) state. Especially the already persisted state should be considered immutable. Instances of `PolarisEntityCore` are also cached via `org.apache.polaris.core.persistence.cache.EntityCacheEntry#entity`.

`PolarisEntityCore` + `PolarisBaseEntity` type hierarchy exposes _public_ setters for effectively all properties. This makes it very easy to (accidentally?) _modify_ entity instances that are cached or in any other way shared. Although the types have some "copy constructors", the risk of modifying attributes is still there. Even if a legit change to a property of a shared object would not be correct, given the Java memory model guarantees.

### To Reproduce

_No response_

### Actual Behavior

_No response_

### Expected Behavior

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### Additional context

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### System information

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Start by reviewing the PolarisEntityCore and PolarisBaseEntity hierarchies, their copy constructors, and the EntityCacheEntry#entity usage in the persistence cache. Determine and document the scope of the immutability change before selecting an implementation; done means shared persisted entities cannot be accidentally modified through the exposed API.

Written by the indexing model from the issue text.