[release/6.0] Move OpenAPI extensions to routing assembly (#36310)

* Move OpenAPI extensions to routing assembly

* Address feedback from code and API review

* Clean up ReadResponseMetadata implementation

* Update IProducesResponseTypeMetadata to expose ContentTypes

* Address more feedback from review

* Update ContentTypes to IEnumerable

* Make IEnumerable non-nullable

* One more feedback

* Address feedback from review

Co-authored-by: Safia Abdalla <safia@microsoft.com>