[feat][csharp][unity] Add unity 2019 support and update samples

[tobocop2]

Summary

This PR adds support Unity 2019+ with some small updates to the api client and exception classes. The code here is also introducing a new awaiter extension in the Unity WebRequest library. This allows the generated client code to use concise and idiomatic await asyncOp syntax even in older Unity versions like 2019 LTS, where UnityWebRequestAsyncOperation does not natively support awaiting.

Motivation

I could not generate a client using Unity 2019 and the changes here have enabled me to generate openapi clients for the version of Unity that I am stuck with. I figured that this would also be very helpful to anyone else in a similar position.

Changes

• Added a new supporting file: UnityWebRequestAwaiterExtension.mustache
• Added preprocessor code to make the exception code and api client code compatible with older versions
• Registered the supporting file in the UnityWebRequest block of CSharpClientCodegen.java
• Ensured no runtime changes for Unity 2020+ (no regressions)
• Included a comment in the generated extension explaining its rationale and version scope

Validation

• Built openapi-generator-cli from source with changes
• Regenerated C# client using -l csharp --library unityWebRequest
• Verified the generated client builds and runs under Unity 2019.4.40f1
• Confirmed await request.SendWebRequest(); compiles and functions as expected

---

PR checklist

• Read the [contribution guidelines](https://github.com/openapitools/openapi-generator/blob/master/CONTRIBUTING.md).

• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work.

• Run the following to [build the project](https://github.com/OpenAPITools/openapi-generator#14---build-projects) and update samples:

  ./mvnw clean package || exit
  ./bin/generate-samples.sh ./bin/configs/csharp-unityWebRequest-netstandard2.0.yaml || exit
  ./bin/generate-samples.sh ./bin/configs/csharp-unityWebRequest-net9.yaml || exit
  ./bin/utils/export_docs_generators.sh || exit
  

• File the PR against the correct branch: master (upcoming 7.x.0 minor release - breaking changes with fallbacks)

• If your PR is targeting a particular programming language, @mention the [technical committee](https://github.com/openapitools/openapi-generator/#62---openapi-generator-technical-committee) members:

  @mandrean
  @shibayan
  @Blackclaws
  @iBicha

[wing328]

thanks for the PR

let's give it a try