Added support for inheritance in Swashbuckle in ASP.NET Core

[knom]

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FYI: @mandrean @frankyjuang @shibayan @Blackclaws @lucamazzanti @wing328

Added the support for polymorphism in models to ASP.NET Core generator.
So that Swashbuckle SwaggerGen re-outputs a correct swagger schema with ALLOF.

[wing328]

LGTM.

[wing328]

FYI. It turns out this feature only available in swashbuckle 6.0+ so I've update swashbuckle version to the latest (6.4.0) via #12965.