Configurable spec loading/parsing

[jimschubert]

Description

We currently rely heavily on swagger-parser to load and parse input specifications. We should put this behind a generic interface (supporting spec location, auth/security inputs), such that users may extend this to their liking.

Suggest a fix/enhancement

If done after #843, we could provide a service located transformer for the generic type mentioned in the interface above to get from the parsed output to our target generic API definition.

It would be nice to include a CLI option to override these (loader, transformer) types.