[csharp] fix enum serialization of first value

[jimschubert]

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: 3.0.0 branch for changes related to OpenAPI spec 3.0. Default: master.
• Copied the technical committee to review the pull request if your PR is targeting a particular programming langauge.

Description of the PR

NOTE: This is a continuation of #6851. opened separately to facilitate discussion of whether this is the best approach, and to verify that I haven't missed anything.

The EmitDefaultValue=false for string based enums will prevent the first
enum value from being serialized, because as 0 it is considered the
default.

This commit assigns an explicit numerical value to all non-integer
enums. This assignment has no effect on the
serialization/deserialization values, and only assigns the compiled
integer.

NOTE: This will have an effect of requiring recompilation of any code
that references the client/server models. This is because:

public enum Pet { Available }

Source files referencing Pet.Available as defined above will have a
constant 0 in place of the enum value.

public enum Pet { Available = 1 }

Source files referencing Pet.Available as defined above will have a
constant 1 in place of the enum value.

After compilation, Pet.Available in both instances lose their semantic
values and refer to the byte representation of their integral values.

For more info, see
https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/keywords/enum

/cc @mandrean
/cc @wing328 (this change is a compile-time breaking change, see above)

[jimschubert]

This PR resolves the issue present in the bug…

Test case:

swagger: "2.0"
info:
  description: "This is a sample server Petstore server.  You can find out more about     Swagger at [http://swagger.io](http://swagger.io) or on [irc.freenode.net, #swagger](http://swagger.io/irc/).      For this sample, you can use the api key `special-key` to test the authorization     filters."
  version: "1.0.0"
  title: "Swagger Petstore"
  termsOfService: "http://swagger.io/terms/"
  contact:
    email: "apiteam@swagger.io"
  license:
    name: "Apache 2.0"
    url: "http://www.apache.org/licenses/LICENSE-2.0.html"
host: "petstore.swagger.io"
basePath: "/v2"
tags:
- name: "pet"
  description: "Everything about your Pets"
  externalDocs:
    description: "Find out more"
    url: "http://swagger.io"
- name: "store"
  description: "Access to Petstore orders"
- name: "user"
  description: "Operations about user"
  externalDocs:
    description: "Find out more about our store"
    url: "http://swagger.io"
schemes:
- "http"
paths:
  /pet:
    post:
      tags:
      - "pet"
      summary: "Add a new pet to the store"
      description: ""
      operationId: "addPet"
      consumes:
      - "application/json"
      - "application/xml"
      produces:
      - "application/xml"
      - "application/json"
      parameters:
      - in: "body"
        name: "body"
        description: "Pet object that needs to be added to the store"
        required: true
        schema:
          $ref: "#/definitions/Pet"
      responses:
        405:
          description: "Invalid input"
definitions:
  PetStatus:
    type: "string"
    description: "pet status in the store"
    enum:
    - "available"
    - "pending"
    - "sold"
  Pet:
    type: "object"
    required:
    - "name"
    - "status"
    properties:
      name:
        type: "string"
        example: "doggie"
      status:
        $ref: "#/definitions/PetStatus"

CLI options:

generate -l csharp -o out -i swagger.yaml

Evaluate:

$ csharp -r:bin/IO.Swagger.dll -r:bin/Newtonsoft.Json.dll -r:bin/RestSharp.dll
Mono C# Shell, type "help;" for help

Enter statements below.
csharp> using IO.Swagger.Model;
csharp> var pet = new Pet("Kitty McKittyface", PetStatus.Available).ToJson();
csharp> pet
"{
  "status": "available",
  "name": "Kitty McKittyface"
}"