Support reftable format

[93578237]

I could not find any issues requesting reftable support.

[ilyagr]

We could make an issue, but basically it has to be done in gitoxide first.

#9336 (comment)

GitoxideLabs/gitoxide#109

A seemingly unsuccessful attempt at a workaround: #8130 (I have neither tested not attempted to review it nor even think about the feasibility of that approach)

[93578237]

I created #9358 to track progress