feat(rust,python): Implement DataFrame.unique(keep="none")

[stinodego]

Partially addresses #6125

Changes:

• Add a KeepNone entry to the KeepStrategy enum
• Implement unique for this option using select -> is_unique -> filter. This should preserve order so the maintain_order argument is ignored.
  • Not sure if this is most efficient - but the existing groupby -> agg_first logic does not seem to be easily adjusted for dropping all duplicates.
• Update things accordingly on the Python side.