Make UnaryPredicate JSON serializable and add unit test

Author: Aniketsy

pyiceberg/expressions/__init__.py

@@ -429,10 +429,12 @@ def bind(self, schema: Schema, case_sensitive: bool = True) -> BooleanExpression
     def as_bound(self) -> Type[BoundPredicate[L]]: ...
 
 
-class UnaryPredicate(UnboundPredicate[Any], IcebergBaseModel, ABC):
+class UnaryPredicate(IcebergBaseModel, UnboundPredicate[Any], ABC):
     type: str
     column: str
 
+    model_config = {"arbitrary_types_allowed": True}
+
     def __init__(self, term: Union[str, UnboundTerm[Any]]):
         if isinstance(term, Reference):
             term_name = term.name