accept optional arbitrary fields when attaching an error to a span

[bpholt]

Without this change, it's still possible to attach arbitrary fields onto an event using log (or onto a span itself using put), but for at least the OpenTelemetry backend, that results in the exception being placed on a distinct event on the span from the other fields. Keeping them together seems more semantically sound.

If this is merged, I plan to use it here in log4cats-natchez. That line would probably become something like

maybeThrowable.fold(Trace[F].log(attributes: _*))(Trace[F].attachError(_, attributes: _*))

Arguably, we could also make a very similar addition to the def log(event: String): F[Unit] method, but I think it's less necessary there because AFAICT, for most of the backends, you can achieve the same result by using def log(fields: (String, TraceValue)*): F[Unit] and adding an "event" -> TraceValue.StringValue(event) with the rest of the attributes being set.