Support batch publish

[mdwaud]

A lot of work has gone into this already and you can see the batch encoder {here](https://github.com/sparkmeter/sink/blob/main/lib/sink/connection/protocol/batch.ex).

Remaining Work

• Implement Switch from fixed packet size to varint #1
• Add new Sink "advanced request" and "advanced response" message type(s) to support batch operations
• Determine interface changes to handlers (do we return the whole batch or one event at a time?
• Determine how to structure response of batches with NACKs
• Make batches aware of message ordering