add support for storing u2f challenges

src/Core/Repositories/IU2fRepository.cs

@@ -0,0 +1,13 @@
+using System;
+using System.Collections.Generic;
+using System.Threading.Tasks;
+using Bit.Core.Models.Table;
+
+namespace Bit.Core.Repositories
+{
+    public interface IU2fRepository : IRepository<U2f, int>
+    {
+        Task<ICollection<U2f>> GetManyByUserIdAsync(Guid userId);
+        Task DeleteManyByUserIdAsync(Guid userId);
+    }
+}