[Refactor] Move SQL classes in their own assembly

BTCPayServer.Data/Data/APIKeyData.cs

@@ -0,0 +1,25 @@
+using System;
+using System.Collections.Generic;
+using System.ComponentModel.DataAnnotations;
+using System.Linq;
+using System.Threading.Tasks;
+
+namespace BTCPayServer.Data
+{
+    public class APIKeyData
+    {
+        [MaxLength(50)]
+        public string Id
+        {
+            get; set;
+        }
+
+        [MaxLength(50)]
+        public string StoreId
+        {
+            get; set;
+        }
+
+        public StoreData StoreData { get; set; }
+    }
+}