Add types for Minimal API

[Swimburger]

ASP.NET Core 6 introduced a new way to build web applications using Minimal API.
This doesn't use controllers or action results, but uses the new IResult type.
It is suggested to add extension methods to IResultExtensions so it can be invoked using Results.Extensions.YourExtension.
Here's the guidance I found: https://docs.microsoft.com/en-us/aspnet/core/fundamentals/minimal-apis?view=aspnetcore-6.0#customizing-results

To get this to work, I had to make a couple of decisions which should be up for discussion:

• The types only seemed to be recognized when targeting net6.0. This is probably not acceptable to maintain support for older versions of .NET.
• The types are in a separate namespace Twilio.AspNet.Core.MinimalApi to prevent name collisions.
• I don't see much value in adding the extension to IResultExtensions, it'll make it so the extension method is available at Results.Extensions.TwiML, but we could just create our own static type TwilioResults.TwiML or something similar.

This is how the new code would be used:

app.MapGet("/voice", () =>
{
    var response = new VoiceResponse();
    response.Say("Hello World!");
    return Results.Extensions.TwiML(response);
});
app.MapGet("/message", () =>
{
    var response = new MessagingResponse();
    response.Message("Hello World!");
    return Results.Extensions.TwiML(response);
});

What do y'all think?

Contributing to Twilio

All third-party contributors acknowledge that any contributions they provide will be made under the same open-source license that the open-source project is provided under.

• I acknowledge that all my contributions will be made under the project's license.

[Swimburger]

Couple of updates:

• I learned how to use C# preprocessor conditionals, so we can include these new features while still supporting older versions of .NET.
• For multi-targeting to work with .NET 6, the build script needed to be updated to use the dotnet CLI more
• We'll keep using Results.Extensions.TwiML as it is the recommended way
• Version has been bumped to 5.71.0
• @dprothero is helping me onboard on this project

[Swimburger]

@dprothero what are the next steps to get this towards a mergable state?

[dprothero]

@Swimburger I think it's there!

[Swimburger]

@dprothero this is ready for review, would you like to review it or should I ask someone else?

[dprothero]

Nice work! Lots of yak shaving to be done, but it looks good.