[csharp-netcore] Inherit api

[devhl-labs]

Please merge https://github.com/OpenAPITools/openapi-generator/pull/12160 first.

Adds the ability to inherit the api. This enables library authors to validate server request parameters and process server responses.

PR checklist

• [x] Read the contribution guidelines.
• [x] 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.
• [x] Run the following to build the project and update samples:

  ./mvnw clean package 
  ./bin/generate-samples.sh
  ./bin/utils/export_docs_generators.sh
  

  Commit all changed files. This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master. These must match the expectations made by your contribution. You may regenerate an individual generator by passing the relevant config(s) as an argument to the script, for example ./bin/generate-samples.sh bin/configs/java*. For Windows users, please run the script in Git BASH.
• [x] If your PR is targeting a particular programming language, @mention the technical committee members, so they are more likely to review the pull request.

@mandrean (2017/08) @frankyjuang (2019/09) @shibayan (2020/02) @Blackclaws (2021/03) @lucamazzanti (2021/05)

[devhl-labs]

I'm no longer using many of the default templates from the rest sharp library, so it may be time to move this library to another generator. This can be another pr though.

Update the PR so there are now two api files that get created in two different namespaces. Org.OpenAPITools.IDefaultApi Interfaces Org.OpenAPITools.BaseApi Performs rest requests Org.OpenAPITools.DefaultApi Generated only one time. Manually edit to perform any parameter validation and server response post processing