Add support for using Cancellation Tokens

[christianhelle]

The changes here are requested by @kirides and will resolve #19

USAGE:
    refitter [URL or input file] [OPTIONS]

EXAMPLES:
    refitter ./openapi.json
    refitter https://petstore3.swagger.io/api/v3/openapi.yaml
    refitter ./openapi.json --namespace "Your.Namespace.Of.Choice.GeneratedCode" --output ./GeneratedCode.cs
    refitter ./openapi.json --namespace "Your.Namespace.Of.Choice.GeneratedCode" --internal
    refitter ./openapi.json --output ./IGeneratedCode.cs --interface-only
    refitter ./openapi.json --use-api-response
    refitter ./openapi.json --cancellation-tokens

ARGUMENTS:
    [URL or input file]    URL or file path to OpenAPI Specification file

OPTIONS:
                                      DEFAULT                                                                    
    -h, --help                                         Prints help information                                   
    -n, --namespace                   GeneratedCode    Default namespace to use for generated types              
    -o, --output                      Output.cs        Path to Output file                                       
        --no-auto-generated-header                     Don't add <auto-generated> header to output file          
        --interface-only                               Don't generate contract types                             
        --use-api-response                             Return Task<IApiResponse<T>> instead of Task<T>           
        --internal                                     Set the accessibility of the generated types to 'internal'
        --cancellation-tokens                          Use cancellation tokens                                   

Using the following OpenAPI spec:

openapi: '3.0.0'
info:
  version: 'v1'
  title: 'Test API'
servers:
  - url: 'https://test.host.com/api/v1'
paths:
  /jobs/{job-id}:
    get:
      tags:
      - 'Jobs'
      summary: 'Get job details'
      description: 'Get the details of the specified job.'
      parameters:
        - in: 'path'
          name: 'job-id'
          description: 'Job ID'
          required: true
          schema:
            type: 'string'
      responses:
        '200':
          description: 'successful operation'
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/JobResponse'
components:
  schemas:
    JobResponse:
      type: 'object'
      properties:
        job:
          type: 'object'
          properties:
            start-date:
              type: 'string'
              format: 'date-time'
            details:
              type: 'string'

Setting the --cancellation-tokens argument will generate code for the following interface:

using Refit;
using System.Threading;
using System.Threading.Tasks;
using System.Collections.Generic;

namespace GeneratedCode
{
    internal interface ITestAPI
    {
        /// <summary>
        /// Get the details of the specified job.
        /// </summary>
        [Get("/jobs/{job-id}")]
        Task<JobResponse> Jobs([AliasAs("job-id")] string job_id, CancellationToken cancellationToken = default);
    }
}