csharp: Add support for generating class structure

Description

I'm using the csharp code generator - I have quite a number of classes that are generated using the c# openapi generator. These generate names like "Company.Project.Models.Area.Entity" which are reasonably long. When codegen runs, it generates these all into a single "Models" folder with very long class names like "CompanyProjectModelsAreaEntity" which doesn't make sense for us. Is there any way to get the codegen to "rehydrate" the folder structure? Generating different swagger.json for example?

Swagger-codegen version

2.2.1

Swagger declaration file content or url

https://gist.github.com/evolvedlight/3db29eb489ab3af509480399d2e63510

Command line used for generation

java -jar ../swagger-codegen-cli-2.2.1.jar generate -i ../swagger.json -l csharp -DpackageName=Company.Namespace

Steps to reproduce

Run the generator. Notice that the class name generated is CompanyVendorProjectFolder1Folder2AnimalsPet instead of creating a nice csharp folder hierarchy

Related issues/PRs

Suggest a fix/enhancement

Someone tells me that this is possible, or is undesirable, or it would be liked and a patch welcomed.

Start with the C# generator behavior described in the issue, using the supplied Swagger gist and the swagger-codegen 2.2.1 command to reproduce the flattened class names. Determine the desired namespace-to-folder mapping and whether the generator should create that hierarchy; done would mean generated classes appear in the requested C# folder structure rather than one Models folder.