Changed domains, configured links between VisaApplication and Applicant,created generic repository and repositories for domains

SchengenVisaApi/Infrastructure/Database/GeneralExceptions/EntityNotFoundException.cs

@@ -0,0 +1,4 @@
+namespace Infrastructure.Database.GeneralExceptions
+{
+    public class EntityNotFoundException<T>(Guid id) : Exception($"Entity {typeof(T).Name} with id '{id}' not found");
+}