Make credential store awaitable

The storage mechanism for credentials is very likely to be an async data
store. So might as well play it safe and make it awaitable.

Octokit/Http/ICredentialStore.cs

@@ -1,11 +1,10 @@
 using System.Diagnostics.CodeAnalysis;
+using System.Threading.Tasks;
 
 namespace Octokit.Internal
 {
     public interface ICredentialStore
     {
-        [SuppressMessage("Microsoft.Design", "CA1024:UsePropertiesWhereAppropriate", 
-            Justification = "The credential store migth not be immediate")]
-        Credentials GetCredentials();
+        Task<Credentials> GetCredentials();
     }
 }