Add DefaultContentType to OpenApiReaderSettings

[LucGenetier]

Is your feature request related to a problem? Please describe.
You recently changes the default content type from 'application/json' to 'application/octet-stream' when it is not specified in the swagger/openapi file.

Describe the solution you'd like
Allow the external user to set the default content type he'd like to use.

Describe alternatives you've considered
I haven't found any alternative.

Additional context
RFC 9110, section 8.3 says (MAY is the important part)

A sender that generates a message containing content SHOULD generate a Content-Type header field in that message unless the intended media type of the enclosed representation is unknown to the sender. If a Content-Type header field is not present, the recipient MAY either assume a media type of "application/octet-stream" ([RFC2046], Section 4.5.1) or examine the data to determine its type.

[darrelmiller]

I think we should be able to add this as a property on OpenApiReaderSettings.

[darrelmiller]

@LucGenetier Our devs are deep in the 3.1 implementation, would you mind sending a PR to add this property to the OpenAPIReadersettings and we will get a release out?

[LucGenetier]

@darrelmiller I just looked at the code and don't see how I could access OpenAPIReadersettings where we need to access it.
In ParseOperationWithEmptyProducesArraySetsResponseSchemaIfExists test, OpenApiReaderSettings is never used/its constructor never called (if I create one)