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

[hammer]

Discussed in https://github.com/pystatgen/sgkit/discussions/652

^{Originally posted by patrick-koenig August 19, 2021}
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?