Define segment span attributes

[cleptric]

We currently lack conventions for materialized attributes for child spans.
For example, sentry.segment.name, which is the name of the segment.
SDKs won't emit these attributes, but Relay or the Buffer will do so.

Refs https://github.com/getsentry/sentry/blob/fa8f90ce3a803961826b30d44043f9675ddece58/src/sentry/spans/consumers/process_segments/enrichment.py#L8-L33

[linear]

CON-57 Define segment span attributes