Add support for generating IApiResponse<T> as return types

[christianhelle]

Add support for generating interfaces that returns Task<IApiResponse<T>> as the return type

Like this:

[Get("/pet/{petId}")]
Task<IApiResponse<Pet>> GetPetById(long petId);

So that if the call to GetPetById() fails then we can handle the negative response ourselves instead of catching an ApiException

var client = RestService.For<ISwaggerPetstore>("https://petstore3.swagger.io/api/v3");
var response = await client.GetPetById(2);

if (!response. IsSuccessStatusCode)
{
    // Do something about the failed response
}

var pet = response.Content;

The IApiResponse<T> interface comes from Refit and is defined as

/// <inheritdoc/>
public interface IApiResponse<out T> : IApiResponse
{
    /// <summary>
    /// Deserialized request content as <typeparamref name="T"/>.
    /// </summary>
    T? Content { get; }
}

/// <summary>
/// Base interface used to represent an API response.
/// </summary>
public interface IApiResponse : IDisposable
{
    /// <summary>
    /// HTTP response headers.
    /// </summary>
    HttpResponseHeaders Headers { get; }

    /// <summary>
    /// HTTP response content headers as defined in RFC 2616.
    /// </summary>
    HttpContentHeaders? ContentHeaders { get; }

    /// <summary>
    /// Indicates whether the request was successful.
    /// </summary>
    bool IsSuccessStatusCode { get; }

    /// <summary>
    /// HTTP response status code.
    /// </summary>
    HttpStatusCode StatusCode { get; }

    /// <summary>
    /// The reason phrase which typically is sent by the server together with the status code.
    /// </summary>
    string? ReasonPhrase { get; }

    /// <summary>
    /// The HTTP Request message which led to this response.
    /// </summary>
    HttpRequestMessage? RequestMessage { get; }

    /// <summary>
    /// HTTP Message version.
    /// </summary>
    Version Version { get; }

    /// <summary>
    /// The <see cref="ApiException"/> object in case of unsuccessful response.
    /// </summary>
    ApiException? Error { get; }
}