Move version field into DeltaTableState struct

[houqp]

Description

version is currently a field in DeltaTable. It would make more sense to move this into DeltaTableState since version maps one to one to state.

[explicite]

@houqp I raised draft for this #553

Still no sure:

1. if we should expose whole state on table
2. Ruby/Python we should move object like state to DeltaTable... definitions, or expose only version

Thanks for advice

[houqp]

Thanks @explicite for taking on this task! The general rule of thumb for exposing internal fields as public is to consider whether there is a dependency between the field we want to expose with any other field in the struct. In this case, I think version was the only field that correlates to state, so it should be safe to expose the state field after we moved the version field into it. If a field is exposed in the Rust side, I think it's reasonable to expose it to python/ruby as well if you want to do that.