Add HeaderBlock methods to get Osmosis replication fields

[jake-low]

Adds new methods to HeaderBlock for retrieving the protobuf fields used by Osmosis for replication.

For context, I'm working on a Rust port of OSMExpress. I'm hoping to use your crate for reading .osm.pbf files and converting them into .osmx files, but to do so I need to be able to read these fields.

Thanks!