Adding support for the Device Flow Oauth authentication pattern

[palenshus]

Adds support for the Device flow authentication pattern: https://docs.github.com/en/developers/apps/authorizing-oauth-apps#device-flow

Resolves #2308

Usage:

var github = new GitHubClient(new ProductHeaderValue("test"));
string clientId = "<clientId>";
var request = new OauthDeviceFlowRequest(clientId);
request.Scopes.Add("repo");

var deviceFlowResponse = await github.Oauth.InitiateDeviceFlow(request);
Console.WriteLine(deviceFlowResponse.UserCode);
OpenWebPage(deviceFlowResponse.VerificationUri);
var token = await github.Oauth.CreateAccessTokenForDeviceFlow(clientId, deviceFlowResponse);
Console.WriteLine(token.AccessToken);

[codecov]

Codecov Report

Merging #2310 (662033f) into main (15887d9) will not change coverage.
The diff coverage is 20.00%.

❗ Current head 662033f differs from pull request most recent head fad9a41. Consider uploading reports for the commit fad9a41 to get more accurate results

@@           Coverage Diff           @@
##             main    #2310   +/-   ##
=======================================
  Coverage   65.99%   65.99%           
=======================================
  Files         554      554           
  Lines       14519    14519           
  Branches      857      857           
=======================================
  Hits         9582     9582           
  Misses       4779     4779           
  Partials      158      158           

Impacted Files	Coverage Δ
Octokit.Reactive/Clients/ObservableOauthClient.cs	0.00% <ø> (ø)
Octokit/Clients/OAuthClient.cs	100.00% <ø> (ø)
Octokit/Models/Response/OauthToken.cs	69.23% <0.00%> (ø)
Octokit/Helpers/ApiUrls.cs	97.53% <100.00%> (ø)

[palenshus]

This failure looks unrelated to my change, possibly just an infrastructure hiccup? Can it be re-run?

[palenshus]

/azp run

[nickfloyd]

@palenshus This is a great addition, thank you for taking the time to do it!

I just had a few comments on how you've set up some of the class property's accessibility. I was wondering, were you favoring protected over private because it's the dominant pattern for most of the models in Octokit.net? It seems that most of those now implement ctors in one form or another so that makes it where I think we could change most of the protected access modifiers to the more appropriate private ones.

I didn't see where we needed to use the class as a derived class but I could've just missed something. Let me know your thoughts on the suggestions when you get the chance.

Thanks again for putting in the effort here - it is sincerely appreciated and will result in something that will help our community.

[nickfloyd]

It looks like we are almost there. 3 properties need to have their setters made private and I think we'll be good to go. Thanks again for putting in the work on this!

[nickfloyd]

This looks great, thanks!

[palenshus]

Thanks Nick, it's been a long road, I'm so glad to have this in now!

[nickfloyd]

release_notes: Adds support for the Device Flow Oauth authentication pattern