Define span events to events mapping

[lmolkova]

Changes

Introduces mapping from Span Events to Events (along with open-telemetry/semantic-conventions#954).

This mechanism allows

• instrumentations to emit structured events using APIs available now in their language (such as LLM Introduce per-message structured GenAI events instead of prompt/completion span events semantic-conventions#980).
• to decouple instrumentation from the long-term unification mechanism between different kinds of events
• to provide the backward compatibility story for Span Events (if they are ever deprecated)

The end-to-end case:

1. Instrumentation emits Span Events using existing Trace API.
   • it provides payload in a dedicated attribute (event.data). It's serialized to JSON (Note: "plain string" is a valid JSON)
   • we may define a mapping for severity as well
2. Mapping component (e.g. a span processor or a collector processor) transforms span events to events using the defined mapping.
   • the current version of this PR defines API-level mapping (for in-process component such as span processor)
   • it should be possible to define a mapping to LogRecords and/or on the OTLP level
3. [Out of scope of this PR] Such component can be implemented
   • as a part of OTel SDK Span implementation and used to deprecate Span Events. It can then follow Event sampling rules rather than Span sampling.
   • as an opt-in contrib span processor/collector processor or on the backend. Then it will follow Span sampling decisions

• Related issues SpanEvent and Event through LogRecord should be "unified" #3406
• Related OTEP(s)
• Links to the prototypes (when adding or changing features)
• CHANGELOG.md file updated for non-trivial changes
• spec-compliance-matrix.md updated if necessary

[lmolkova]

Based on the discussions in the Spec and Semconv WG, we'd try to leverage Event API (or log API in the short term) to emit events with body instead of adding mapping to span events.
The would help us to retire span events sooner and invest into Event API where it's lacking.

The attribute like event.data that would be essential for Event API -> log-based events mapping (if it's ever needed).

So I'm going to close this PR.