[otel] add otel.event.name attribute

[pellared]

Fixes #2913

This PR introduces the otel.event.name attribute for use by non-OTLP exporters and logging libraries.

The otel.event.name attribute can be used by:

• Non-OTLP exporters to emit the EventName when the destination does not support EventName or an equivalent field
• Applications using existing logging libraries to add event name information that can be used to set the EventName field by Collector or SDK components

This follows a similar pattern to #2715, but uses the otel.* namespace instead of event.* to properly represent OpenTelemetry-specific concepts used for bridging between formats.

Example from OTel Go:

• sdk/log: update ExampleProcessor_eventName to use otel.event.name attribute opentelemetry-go#7568

[github-actions]

This PR contains changes to area(s) that do not have an active SIG/project and will be auto-closed:

• otel

Such changes may be rejected or put on hold until a new SIG/project is established.

Please refer to the Semantic Convention Areas
document to see the current active SIGs and also to learn how to kick start a new one.

[cijothomas]

LGTM!