Align extension encoding (nested struct vs flat fields) across ETW and user-events exporters

[utpilla]

The ETW and user-events exporters use different encoding styles for Common Schema PartA extensions. The discrepancy is limited to PartA — PartB and PartC are flat in all exporters.

Current state

PartA Field	ETW Logs (nested struct)	User-Events Logs (flat)	User-Events Trace (flat)
trace ID	ext_dt struct → traceId	ext_dt_traceId	ext_dt_traceId
span ID	ext_dt struct → spanId	ext_dt_spanId	ext_dt_spanId
cloud role	ext_cloud struct → role	ext_cloud_role	ext_cloud_role
cloud role instance	ext_cloud struct → roleInstance	ext_cloud_roleInstance	ext_cloud_roleInstance

As per the spec, both forms are valid, but the spec recommends nested structs and does not guarantee agents handle both identically.

Proposal

Align all exporters on the preferred nested struct convention for PartA extensions.

Affected crates:

• opentelemetry-user-events-logs — switch ext_dt_traceId, ext_dt_spanId, ext_cloud_role, ext_cloud_roleInstance to nested structs
• opentelemetry-user-events-trace — same fields

The opentelemetry-etw-logs crate already uses nested structs (after #588) and does not need changes.