fix: Ensure input types are modelled by SwaggerGenerator

[djones6]

Description

Ensures that input types are included in the list of types (models) described in Kitura's generated swagger. I also added a check to avoid duplicating effort to build the same model where the same type appears in multiple routes (a common occurrence).

Motivation and Context

Fixes a bug in the SwaggerGenerator where the generated swagger document could refer to a model that was not described.

This affected routes that have both an input and output type, but no identifier (eg. POST). For such routes, the input type was not added to the list of models, and so the document was invalid if that same type was not also used as an output type.

How Has This Been Tested?

The changes pass the existing tests. I have not added a specific test to cover this case in this PR, but intend to do so in a follow-up one.

Checklist:

• If applicable, I have updated the documentation accordingly.
• If applicable, I have added tests to cover my changes.