Attempts to resolve #961 (#962)

* Move REST requests to appropiate class

* Add call to ClientHelper and expose to public API

* Expose shard count request in public api

* Expose method from interface

* Update sharded client to utilize the new method

* Method is already implemented in a base class

* Refactor name to fit pattern for methods returning a `Task`

* Adds missing ConfigureAwait

* Corrects unnecessary whitespace

* Removes unneeded whitespace

src/Discord.Net.Core/IDiscordClient.cs

@@ -36,5 +36,7 @@ namespace Discord
         Task<IVoiceRegion> GetVoiceRegionAsync(string id, RequestOptions options = null);
 
         Task<IWebhook> GetWebhookAsync(ulong id, RequestOptions options = null);
+
+        Task<int> GetRecommendedShardCountAsync(RequestOptions options = null);
     }
 }