Added RequestOptions to RestClient methods. Added guild summary paging.

src/Discord.Net.Rest/API/Rest/GetGuildSummariesParams.cs

@@ -0,0 +1,9 @@
+#pragma warning disable CS1591
+namespace Discord.API.Rest
+{
+    internal class GetGuildSummariesParams
+    {
+        public Optional<int> Limit { get; set; }
+        public Optional<ulong> AfterGuildId { get; set; }
+    }
+}