Move event to span event bridge from contrib to core

With [spec PR #5006](https://github.com/open-telemetry/opentelemetry-specification/pull/5006), the span to span event bridge I [prototyped in contrib](https://github.com/open-telemetry/opentelemetry-java-contrib/tree/main/processors#event-to-spanevent-bridge) should be promoted to core to live in `opentelemetry-sdk-extension-incubator` (until stable).