Make default swagger content type configurable

[evaline-ju]

Is your feature request related to a problem? Please describe.

In one of the services that used caikit it was discovered that list inputs were not processed correctly, namely the multiple inputs in the list were processed as a conjoined string instead of separate inputs. A public discussion here in fastAPI points to an open issue in swagger: swagger-api/swagger-js#2713

PR #576 had changed the default to swagger-ui content to multipart/form-data. However, currently some services that require list inputs may face problems when users try the api spec out through fast API. For these services we may want to instead provide application/json as the default. If the default swagger content is configurable, caikit users can choose based on their service needs.

Describe the solution you'd like

Configurable swagger default potentially under a key such as runtime.http.swagger_default