[FEAT]: Adding Copilot for Business support (#2826)

* initial tests and implementation of Copilot for Business client API

* updated billing settings documentation

* renames and refactors - clarity and simplified

* using context to ensure license clean up

* extra documentation and used ApiOptions instead of custom class

* implemented observable clients

* Fixing convention issues

* renaming for clarity

---------

Co-authored-by: Nick Floyd <139819+nickfloyd@users.noreply.github.com>

Octokit.Reactive/IObservableGitHubClient.cs

@@ -43,5 +43,6 @@ namespace Octokit.Reactive
         IObservableMetaClient Meta { get; }
         IObservableActionsClient Actions { get; }
         IObservableCodespacesClient Codespaces { get; }
+        IObservableCopilotClient Copilot { get; }
     }
 }