OneTime Invites Server handling (#17)

* add server-side handling for one time invites

MareSynchronosServer/MareSynchronosShared/Models/GroupTempInvite.cs

@@ -0,0 +1,17 @@
+using System;
+using System.Collections.Generic;
+using System.ComponentModel.DataAnnotations;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace MareSynchronosShared.Models;
+
+public class GroupTempInvite
+{
+    public Group Group { get; set; }
+    public string GroupGID { get; set; }
+    [MaxLength(64)]
+    public string Invite { get; set; }
+    public DateTime ExpirationDate { get; set; }
+}