code: public SimpleFieldValidation addRule(ExecutionPath fieldPath, BiFunction<FieldAndArguments, FieldValidationEnvironment, Optional<GraphQLError>> rule) {
          rules.put(fieldPath, rule);
          return this;
      }
docs: Adds the rule against the field address path.  If the rule returns an error, it will be added to the list of errors
  
  @param fieldPath the path to the field
  @param rule      the rule function
  
  @return this validator