[csharp-netcore] Fix C# NetCore constructors generation with snake case

Created by: salimzdn

resolve #14571

This PR resolves the issue by removing camelCase transformation in models constructor for csharp-netcore.

Using this OpenApi specificatin file

{
    "openapi": "3.0.1",
    "info": {
        "title": "My Web API",
        "description": "My Web API",
        "version": "1.22.2"
    },
    "tags": [],
    "paths": {},
    "components": {
        "schemas": {
            "MyTokenType": {
                "type": "string",
                "description": "Token type",
                "enum": [
                    "JWT",
                    "OIDC_ACCESS_TOKEN"
                ],
                "x-enum-descriptions": [
                    "JWT",
                    "OIDC_ACCESS_TOKEN"
                ]
            },
            "MyTokenRequest": {
                "type": "object",
                "description": "My token request",
                "properties": {
                    "$_type": {
                        "type": "string",
                        "default": "MyTokenRequest",
                        "enum": [
                            "MyTokenRequest"
                        ]
                    },
                    "token": {
                        "type": "string"
                    },
                    "type": {
                        "$ref": "#/components/schemas/MyTokenType"
                    }
                }
            }
        }
    }
}

And using this config file

{
    "packageName" : "com.myApi",
    "nullableReferenceTypes" : true,
    "targetFramework" : "net6.0",
    "modelPropertyNaming" : "snake_case"
}

And using this command

npx openapi-generator-cli generate -i swagger_simple_anonymised.json -g csharp-netcore -o out_dotnet -c openApiGeneratorConfig.json

Without the fix, the resulted constructor was

 public MyTokenRequest(TypeEnum? type = TypeEnum.MyTokenRequest, string token = default(string), MyTokenType? type = default(MyTokenType?))
        {
            this._type = type;
            this.token = token;
            this.type = type;
        }

With the fix, the resulted constructor is now

public MyTokenRequest(TypeEnum? _type = TypeEnum.MyTokenRequest, string token = default(string), MyTokenType? type = default(MyTokenType?))
        {
            this._type = _type;
            this.token = token;
            this.type = type;
        }

See issue #14571 to know more

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• 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.
• Run the following to build the project and update samples:

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  ./bin/generate-samples.sh
  ./bin/utils/export_docs_generators.sh

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  ./bin/utils/ensure-up-to-date.sh

  Commit all changed files.
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