ref(consumer): change BytesInsertBatch to support builder pattern

[onewland]

I want to start emitting how many events by type are being processed by the EAP consumer. I'm dividing that up into 3 changes:

1. (this change) change how we build BytesInsertBatch to support a builder pattern (so it's more simple to omit dataset-specific fields)
2. start keeping track of how many items by type are being processed in the EAPItems processor: [wip] feat(eap): record item type counts in batches #7481
3. emit the actual metric at the end of a batch write