fix(openapi): add generated schema to custom request body media types

[bram123]

Q	A
Branch?	5.0 for bug fixes
Tickets	Closes #8661
License	MIT
Doc PR

When providing a custom RequestBody with content on an operation (for example, to configure OpenAPI examples on request payloads), OpenApiFactory previously discarded the automatically generated input schemas..

Now the MediaType is merged with the generated data, when no schema value is set.
If only a subset of request mimeTypes are customized, all expected types get the schema

Added functional test case testRequestBodyExamplesRetainAutoGeneratedSchema (using the RequestBodyWithExamplesResource fixture class), asserting all requested mimetypes are added and have a schema set, with the two customized mimetypes having the schema added to the existing mimetype dataset.

[soyuka]

I'm not sure that this is possible, if you need this put it in a custom openapi factory. I think that when you have a custom request body it's desired that we don't override anything, indeed we can't really predict that you'll answer the output that are configured on "standard" objects.