C# template refactor

PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh and ./bin/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\.
• Filed the PR against the correct branch: master, 4.0.x. Default: master.
• Copied the technical committee to review the pull request if your PR is targeting a particular programming language.

Description of the PR

A refactoring proof of concept for the C# client.

This puts a client abstraction on top of RestSharp, such that consumers can provide a custom implementation. This is useful for SDK creators who would rather use HttpClient, or consumers who would rather construct APIs with their own solution (e.g. in-house api accessors with tracing and logging applied).

This PR includes only template changes, as generating working C# clients is currently broken on master due to a regression in enum processing (see gist).

I'm opening this for feedback.

To evaluate:

cd samples/client/petstore/csharp-refactor/OpenAPIClient
sh build.sh
csharp -r:bin/Org.OpenAPITools.dll -r:bin/Newtonsoft.Json.dll -r:bin/RestSharp.dll

csharp> using Org.OpenAPITools.Api;
csharp> var api = new PetApi("http://petstore.swagger.io/v2")
csharp> api.GetPetById(2)
class Pet {
  Id: 2
  Category: class Category {
  Id: 1
  Name: feline
}

  Name: catty
  PhotoUrls: System.Collections.Generic.List`1[System.String]
  Tags: System.Collections.Generic.List`1[Org.OpenAPITools.Model.Tag]
  Status: Available
}