Default `revision` to latest (highest number in directory) when querying database

It would be nice to default to the highest revision number in the directory

Example: skipping revision in the from_db call would scan this folder and return the vehicle at r2.yaml.

v1/fastsim-3/conv/ford/fusion/2012/base/
  r1.yaml
  r2.yaml

This would enable users to provide just v1/fastsim-3/conv/ford/fusion/2012 as a path, as variant and extension already have sensible defaults base and yaml

For example these paths would all be valid and return the same vehicle:

• v1/fastsim-3/conv/ford/fusion/2012/base/r2.yaml
• v1/fastsim-3/conv/ford/fusion/2012/base/r2
• v1/fastsim-3/conv/ford/fusion/2012/base
• v1/fastsim-3/conv/ford/fusion/2012

Currently only the first two of these work.

This would need to be done in Rust, as thats where 95% of the database loading stuff is done.

Notes:

• Very easy to do with local paths
• More finagling required for remote databases, getting the list of files in the dir, etc.
• What value could Python (specifically the internals of from_db) provide to Rust to signal we should scan for latest?
  • change from supplying a u32 to a more flexible type?
  • just supply u32::MAX aka 4294967295, and check on the rust side when loading?