Migrate guild objects to the patch system

PluralKit.Core/Models/Patch/MemberGuildPatch.cs

@@ -0,0 +1,13 @@
+#nullable enable
+namespace PluralKit.Core
+{
+    public class MemberGuildPatch: PatchObject
+    {
+        public Partial<string?> DisplayName { get; set; }
+        public Partial<string?> AvatarUrl { get; set; }
+
+        public override UpdateQueryBuilder Apply(UpdateQueryBuilder b) => b
+            .With("display_name", DisplayName)
+            .With("avatar_url", AvatarUrl);
+    }
+}