Rewrite `as_polars_series` (for `vctr` package based vectors)

[eitsupi]

Continued from #368.

as_polars_series was added in #369, but I think we should rewrite that.

This issue is related to the Arrow extension type, which Polars does not currently support (pola-rs/polars#9112), but support may be added in the future.

Useful resources.

• apache/arrow-nanoarrow#288
• https://github.com/apache/arrow/blob/f7947cc21bf78d67cf5ac1bf1894b5e04de1a632/r/R/array.R
• https://github.com/apache/arrow/blob/f7947cc21bf78d67cf5ac1bf1894b5e04de1a632/r/R/extension.R#L469