Added interfaces to read and write data in DB and added DbContext implementing these

SchengenVisaApi/Infrastructure/Database/IUnitOfWork.cs

@@ -0,0 +1,9 @@
+namespace Infrastructure.Database
+{
+    public interface IUnitOfWork
+    {
+        /// Save changes in data storage
+        /// <param name="cancellationToken">Cancellation Token</param>
+        Task SaveAsync(CancellationToken cancellationToken);
+    }
+}