Add publishCheckpoint outcome attribute to traces

[phbnf]

While I'm there:

• rename error.type to tessera.publishCheckpoint.outcome
• Make outcomeAttr an attribute.KeyValue instead of a slice of such
• Introduce two new attribute values, success and aborted

This will:

• provide a better attribute name since for instance "skipped" is not an error
• avoid empty attributes, which are not very well supported by GCP (it shows a long hex string instead)
• add the outcome to traces

[phbnf]

As per https://opentelemetry.io/docs/specs/semconv/general/recording-errors/#recording-errors-on-spans, we should set error.Type. However, we don't put it in other places where we use traces, so we should probably do a cleanup everywhere.