Add `space-fsaverage` to `desc-reg` sphere files without space entity

[tsalo]

A short summary of what you would like to see in NiBabies.

I've seen outputs from Nibabies with the following naming convention: <source_entities>_hemi-<L|R>_desc-reg_sphere.surf.gii. Based on a discussion with @effigies, it looks like this is a registration sphere from fsnative to fsaverage, so it would be clearer to include space-fsaverage in the filename.

Please correct me if that's not what this file is.

Do you have any interest in helping implement the feature?

Yes!

Add any additional information or context about the request here.

No response

[tsalo]

@mgxd I'm interested in implementing this. Should I target master or docker/fit-apply?

[mgxd]

hey @tsalo - awesome! docker/fit-apply merge is imminent, so definitely that one