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

test/e2e-cypress/static/documents/features/petstore-only-pet.openapi.yaml

@@ -415,4 +415,4 @@ components:
     api_key:
       type: apiKey
       name: api_key
-      in: header
+      in: header