Add nullable annotation support to AspNetCoreServer

[janssen92]

Since c# 8 / .net core 3.0 it is possible to add a nullable annotation to a project file.
This allows to use the ? annotation not only on non-nullable types but on all types.

Using this annotations can improve the readability within the code and also improves API behavior in regards to optional (nullable) properties.

The default behavior is not altered, it should stay compatible with everything.

Tests were added to ensure correct behavior.

This PR is a possible solution for:
#9190
#9022

@shibayan @Blackclaws @lucamazzanti

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[devhl-labs]

My pr has this switch in the abstract.

#9235

[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

[janssen92]

@wing328 Thanks, I updated the commit :)

@devhl-labs I adjusted my config/variable naming to be compatible with your PR #9235

[wing328]

@devhl-labs thanks for reviewing and your work on #9235 to kick-start the work on nullable annotation support.

@janssen92 thanks for PR as well as the test.

Merging this one as it has tests to cover the change.

[devhl-labs]

Isnt the switch in the wrong file though? This is only for the asp net core geneeator, not the broader csharp language. This doesnt replace my pr.

[janssen92]

@devhl-labs aggreed, i wanted to stay compatible to your PR. It can be moved to the abstract part at any time, so there shouldn't be a problem.