Add ReadWriteLogRecord accessors

[jack-berg]

Currently, ReadWriteLogRecord forces you to make an immutable copy of the data (by calling toLogRecordData()) if you want to access any of the fields.

In cases like the EventToSpanEventBridge, this causes unnecessary allocations in cases where log records are not events.

Let's followup on this long time TODO and add dedicated accessors. Minimally, the ones that are needed for the EventToSpanEventBridge use case.