Add missing fields to Milestone class

[StanleyGoldman]

Adding missing Milestone fields as per #1358

[ryangribble]

Thanks for the PR 👍

Adding the HtmlUrl is 💎 - this is the url to this milestone on github.com and definitely should be included

But fields like LabelsUrl generally wouldn't be added to octokit.net, since we aim to provide a c# model over the github API, we don't normally expose the internal API hypermedia url links to the end user.

Instead we provide access to the associated API endpoint through the object model, in this case it's at client.Issues.Labels.GetAllForMilestone()

Given the above, are you cool to drop off that labels field from the PR? If not let us know your use case and we can discuss 😀

[StanleyGoldman]

Done

[ryangribble]

Sorry for the delay, have been having a bit of no home computer time this week :)

Thanks for the PR!