Closes#3243.
This PR exposes a new public api `Permutation.take_offsets(offsets:
list[int])`, since users initially had to call __getitems__ directly to
batch-fetch rows by position.
Currently, the name matches the existing `Table.take_offsets` pattern,
and now the dunder `__getitem__` and `__getitems__` now delegate to it.
Also, fixes a parse error when `PermutationReader::take_offsets` gets an
empty list. Now returns an empty `RecordBatch` with the correct schema
instead. Bundled this because without the fix the new public API blows
up on a perfectly reasonable input.
`__getitems__` is preserved since PyTorch's batched DataLoader requires
it.
### Testing
- Added 3 new Rust tests for empty offsets including permutation table
with Select::All, Select::Columns, and identity path
- Added 3 new Python tests for the public API including a happy case,
and empty input on both identity and permutation
clippy, format, check all clean!
cc: @westonpace
