This commit allows logging errors in a slog.Logger injected in the
context. This type of logger is not currently used directly in step-ca,
but this will change in the future.
Subject Common Names can now also be configured to be allowed or
denied, similar to SANs. When a Subject Common Name is not explicitly
allowed or denied, its type will be determined and its value will be
validated according to the constraints for that type of name (i.e. URI).
