Databento-dbn

- Added Python DBZ writing example
- Changed [databento-defs](https://crates.io/crates/databento-defs) dependency to
crates.io version

Enhancements
- Added interface for writing DBZ files
- Enabled Zstd checksums
- Changed DBZ decoding to use [streaming-iterator](https://crates.io/crates/streaming-iterator)

Breaking changes
- Changed JSON output to NDJSON

Bug fixes
- Change nanosecond timestamps to strings in JSON to avoid loss of precision when parsing

- Initial release