Implement pfs-sky transformation?

It was discussed a while ago whether `pfi-sky` transformation is needed or not. This transformation is thought to be needed to look for guide stars by projecting PFI-AG regions on the sky.   At that time, we thought it will work querying slightly wider region than AGs and picking the stars which will be on the AG.

Recently, however, Martin found that `pfi-sky` transformation is still needed after some development work. The below is a comment from Martin Reinecke in INSTRM-1042

Sorry to come back to this, but I just realized that I will most likely need the `pfi_sky` direction as well. Without access to this functionality, I'd have to query a region larger than a square degree when looking for guide star candidates, which will result in very large query results and may be rejected by the DB servers.

A rough approximation is currently sufficient for my purposes, so I can try to implement a first-order accurate solution as a workaround. But I'd still be happy of there was an official way to compute these transforms as well.

Is the `pfi_sky` transformation direction currently planned, and if so, what are the (rough) time scales?