engine-v3.0.0-preview.19

• Changed implementation of EntityStore.CloneEntity()

  Old: Used JSON serialization as fallback if source entity has one or more non-blittable component types.
  E.g. a struct component containing fields with reference types like a array, List<> or Dictionary<,>.

  New: Non-blittable components are now cloned using a static CopyValue() method with must part of component type.
  E.g.

  public struct MyComponent : IComponent
  {
      public int[]   array;
    
      static void CopyValue(in MyComponent source, ref MyComponent target, in CopyContext context) {
          target.array = source.array?.ToArray();
      }
  }

  In case CopyValue() is missing an exception is thrown including the required method signature in the exception message.
  Info: Removing JSON serialization from cloning has additional advantages.

  1. Much faster as JSON serialization/deserialization is expensive.
  2. No precision loss when using floating point types.