Streaming exporter

Fixes #125
Closes #177

DiscordChatExporter.Core.Services/Extensions.cs

@@ -0,0 +1,22 @@
+using System.Collections.Generic;
+using System.Runtime.CompilerServices;
+using System.Threading.Tasks;
+
+namespace DiscordChatExporter.Core.Services
+{
+    public static class Extensions
+    {
+        private static async ValueTask<IReadOnlyList<T>> AggregateAsync<T>(this IAsyncEnumerable<T> asyncEnumerable)
+        {
+            var list = new List<T>();
+
+            await foreach (var i in asyncEnumerable)
+                list.Add(i);
+
+            return list;
+        }
+
+        public static ValueTaskAwaiter<IReadOnlyList<T>> GetAwaiter<T>(this IAsyncEnumerable<T> asyncEnumerable) =>
+            asyncEnumerable.AggregateAsync().GetAwaiter();
+    }
+}