add support for storing u2f challenges

src/Sql/dbo/Tables/U2f.sql

@@ -0,0 +1,11 @@
+CREATE TABLE [dbo].[U2f] (
+    [Id]           INT              NOT NULL IDENTITY,
+    [UserId]       UNIQUEIDENTIFIER NOT NULL,
+    [KeyHandle]    VARCHAR (50)     NOT NULL,
+    [Challenge]    VARCHAR (50)     NOT NULL,
+    [AppId]        VARCHAR (50)     NOT NULL,
+    [Version]      VARCHAR (50)     NOT NULL,
+    [CreationDate] DATETIME2 (7)    NOT NULL,
+    CONSTRAINT [PK_U2f] PRIMARY KEY CLUSTERED ([Id] ASC),
+    CONSTRAINT [FK_U2f_User] FOREIGN KEY ([UserId]) REFERENCES [dbo].[User] ([Id])
+);