pass through descriptions alongside interfaces

[hanakslr]

ReadMe and the OpenApi spec as well as a maintainer all say that as of 3.1 it is supported to provide a description alongside a $ref.

Additionally, as of OpenAPI 3.1, you can include summary and description fields in addition to the $ref pointer, which will override the summary and description fields of the referenced component. Everything else will be fully deferenced.

I modified the code to pass through the description field for interfaces. Any other fields will be rejected and error. This enables use to just use a description field instead of the union workaround below, which was causing issues with the generated client library.

schema:
     allOf:              
              - $interface: "IdentitySets#SameAsInterface"
              - type: "object"
                description: "test ref description"

can now be:

schema:
     $interface: "IdentitySets#SameAsInterface"
     description: "test ref description"

I verified that the generated code renders as expected in Readme and plays nicely with openapi-typescript. This resolves these two issues that Blake reported: #3 #2

[emk]

Looks great!