code: protected Specification<ENTITY> buildStringSpecification(StringFilter filter, SingularAttribute<? super ENTITY,
          String> field) {
          return buildSpecification(filter, root -> root.get(field));
      }
docs: Helper function to return a specification for filtering on a {@link String} field, where equality, containment,
  and null/non-null conditions are supported.
  
  @param filter the individual attribute filter coming from the frontend.
  @param field  the JPA static metamodel representing the field.
  @return a Specification