Expose array offsets and lengths in `geobuffers()` output

[paleolimbot]

Currently, calling geobuffers() on a sliced array will result in buffers that don't necessarily line up with the logical content of the array (i.e., non-zero offsets are not supported). This is consistent with pyarrow.Array.buffers but is confusing since there is no way to check the "all offsets are 0" assumption or get the information required to apply the offsets. This should be implemented by another method (maybe geoslices()?).