feat(RequestBody): validation support for required fields (#6223)

fixes #5181

* application/json
* application/xml
* application/x-www-form-urlencoded

* Set requestBodyValue values to be an immutable Map, as "value". Previously stored as a normal String.
* This enables adding "errors" to the Map, for validation use

* note: getOAS3RequiredRequestBodyContentType requires state.spec,
* which is not available to state.oas3

src/core/components/operation.jsx

@@ -192,6 +192,8 @@ export default class Operation extends PureComponent {
                     operation={ operation }
                     specActions={ specActions }
                     specSelectors={ specSelectors }
+                    oas3Selectors={ oas3Selectors }
+                    oas3Actions={ oas3Actions }
                     path={ path }
                     method={ method }
                     onExecute={ onExecute } />