[csharp][aspnetcore] Allow the use of endpoints (single class per operation)

Created by: Blackclaws

This adds support for endpoints, i.e. a single class per operation to the generator. Unfortunately the way the generator currently works they are still all part of the same file. We can discuss whether we want to put them under an even further nested namespace. As a side effect enabled actual usage of annotations. Might want to expand the annotations of the normal controllers later on as well.

PR checklist

• Read the contribution guidelines.
• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
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  ./mvnw clean package 
  ./bin/generate-samples.sh
  ./bin/utils/export_docs_generators.sh

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