Add support for generating 'internal' types

[christianhelle]

The changes here are requested by @kirides and will resolve #20

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

EXAMPLES:
    refitter ./openapi.json --namespace "Your.Namespace.Of.Choice.GeneratedCode" --output ./Output.cs

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
                                                       (default: public)

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 --internal argument will generate code for the following :

using Refit;
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);
    }
}

namespace GeneratedCode
{
    using System = global::System;   

    [System.CodeDom.Compiler.GeneratedCode("NJsonSchema", "13.18.2.0 (NJsonSchema v10.8.0.0 (Newtonsoft.Json v13.0.0.0))")]
    internal partial class JobResponse
    {

        [System.Text.Json.Serialization.JsonPropertyName("job")]

        [System.Text.Json.Serialization.JsonIgnore(Condition = System.Text.Json.Serialization.JsonIgnoreCondition.WhenWritingDefault)]   
        public Job Job { get; set; }

        private System.Collections.Generic.IDictionary<string, object> _additionalProperties;

        [System.Text.Json.Serialization.JsonExtensionData]
        public System.Collections.Generic.IDictionary<string, object> AdditionalProperties
        {
            get { return _additionalProperties ?? (_additionalProperties = new System.Collections.Generic.Dictionary<string, object>()); }
            set { _additionalProperties = value; }
        }

    }

    [System.CodeDom.Compiler.GeneratedCode("NJsonSchema", "13.18.2.0 (NJsonSchema v10.8.0.0 (Newtonsoft.Json v13.0.0.0))")]
    internal partial class Job
    {

        [System.Text.Json.Serialization.JsonPropertyName("start-date")]

        [System.Text.Json.Serialization.JsonIgnore(Condition = System.Text.Json.Serialization.JsonIgnoreCondition.WhenWritingDefault)]   
        public System.DateTimeOffset StartDate { get; set; }

        [System.Text.Json.Serialization.JsonPropertyName("details")]

        [System.Text.Json.Serialization.JsonIgnore(Condition = System.Text.Json.Serialization.JsonIgnoreCondition.WhenWritingDefault)]   
        public string Details { get; set; }

        private System.Collections.Generic.IDictionary<string, object> _additionalProperties;

        [System.Text.Json.Serialization.JsonExtensionData]
        public System.Collections.Generic.IDictionary<string, object> AdditionalProperties
        {
            get { return _additionalProperties ?? (_additionalProperties = new System.Collections.Generic.Dictionary<string, object>()); }
            set { _additionalProperties = value; }
        }
    }
}