Make default swagger content type configurable

## 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](https://github.com/tiangolo/fastapi/discussions/8741) points to an open issue in swagger: https://github.com/swagger-api/swagger-js/issues/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`