Unify boxed and primitive types of metadata tables exposed in api

[jayhan94]

Currently, there's inconsistent usage of primitive and boxed types that needs unification.
For example:

public class TableColumnImpl implements TableColumn, Serializable {
    private final long columnId;
    private final String columnName;
    private final DataType columnType;
    private final Boolean isNullable; // Should be `boolean`? But consider null scenarios
    private final Object defaultValue;
    private final Object initialDefault;
    private final TableColumnStatistics columnStatistics;
  
    // ......
}

The isNullable field uses Boolean where boolean would typically be preferred. However, before changing this, we must evaluate whether the field needs to represent null semantics. If three-state logic (true/false/null) is required, Boolean remains appropriate, otherwise, boolean should be used for better performance and null-safety.