Port GFF3 I/O methods of scikit-allel to sgkit?

### Discussed in https://github.com/pystatgen/sgkit/discussions/652

<div type='discussions-op-text'>

<sup>Originally posted by **patrick-koenig** August 19, 2021</sup>
Hi sgkit team,

is it planned to port the GFF3 I/O utility methods (especially the gff3_to_dataframe() method) of scikit-allel to sgkit?</div>