Ensure that requiredOptions work within imperativeConfiguration

[nickevansuk]

Check that requiredOptions works within imperativeConfiguration, e.g. Order.json#L110

[henryaddison]

I believe this is already covered by RequiredOptionalFieldsRule - https://github.com/openactive/data-model-validator/blob/master/src/rules/core/required-optional-fields-rule.js#L37

See specs that test this https://github.com/openactive/data-model-validator/blob/master/src/rules/core/required-optional-fields-rule-spec.js#L22-L36 and https://github.com/openactive/data-model-validator/blob/master/src/rules/core/required-optional-fields-rule-spec.js#L140-L181