Add nullable annotation support to AspNetCoreServer

Created by: 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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