Add super basic group model/command

PluralKit.Core/Models/PKGroup.cs

@@ -0,0 +1,17 @@
+using NodaTime;
+
+#nullable enable
+namespace PluralKit.Core
+{
+    public class PKGroup
+    {
+        public GroupId Id { get; }
+        public string Hid { get; } = null!;
+        public SystemId System { get; }
+
+        public string Name { get; } = null!;
+        public string? Description { get; }
+        
+        public Instant Created { get; }
+    }
+}