[aspnetcore] Improve model inheritance support by including JsonSubTypes to properly handle serialization

[goexplore]

Generator aspnetcore mustache templates is missing a mechanism that can support polymorphism(inheritance).
Serializing child types will end up as their base class without indicating known types to the base class.

This pull request will also be a fix for #7377

• add package reference on JsonSubTypes in `Project.csproj.mustache
• apply usage by including KnownSubTypes and discriminator in model.mustache

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[wing328]

Thanks for the PR but your commit (as shown in the Commits tab) is not linked to your Github account, which means this PR won't count as your contribution in https://github.com/OpenAPITools/openapi-generator/graphs/contributors.

Let me know if you need help fixing it.

Ref: https://github.com/OpenAPITools/openapi-generator/wiki/FAQ#how-can-i-update-commits-that-are-not-linked-to-my-github-account

[goexplore]

@wing328, thanks for your review.
Any chance to review this pull request again? Account is now linked to the commit

[wing328]

cc C# tech committee: @mandrean (2017/08) @frankyjuang (2019/09) @shibayan (2020/02)

[wing328]

LGTM but I've not tested it locally

[wing328]

@goexplore thanks for the PR, which has been included in the v5.1.0 release: https://twitter.com/oas_generator/status/1373636654024380423