feat: add Redis identify ratelimiter

PluralKit.Core/Services/RedisService.cs

@@ -0,0 +1,14 @@
+using StackExchange.Redis;
+
+namespace PluralKit.Core;
+
+public class RedisService
+{
+    public ConnectionMultiplexer Connection { get; set; }
+
+    public async Task InitAsync(CoreConfig config)
+    {
+        if (config.RedisAddr != null)
+            Connection = await ConnectionMultiplexer.ConnectAsync(config.RedisAddr);
+    }
+}