Asymmetry with DeltaTablePartition and actions

[rtyler]

While upgrading a project from 0.16.x to the newer Rust versions, I noticed that our models (Add, Remove) all expect partition_values as HashMap<String, Option<String>>

This is what DeltaTablePartition used prior to some arrow refactoring led by @roeap

I think the asymmetry should probably be addressed by updating the action models, e.g. here