Implement CurrentUserService

[eimerreis]

Obtain UserId from AuthenticationMiddleware, so that EntityFramework can write Audit Information to Database

 public override Task<int> SaveChangesAsync(CancellationToken cancellationToken = new CancellationToken())
        {
            foreach (var entry in ChangeTracker.Entries<AuditableEntity>())
            {
                switch (entry.State)
                {
                    case EntityState.Added:
                        entry.Entity.CreatedBy = _CurrentUserService.UserId;
                        entry.Entity.Created = DateTime.Now;
                        break;
                    case EntityState.Modified:
                        entry.Entity.LastModifiedBy = _CurrentUserService.UserId;
                        entry.Entity.LastModified = DateTime.Now;
                        break;
                }
            }

            return base.SaveChangesAsync(cancellationToken);
        }