Allow space-fsaverage for sphere files

[tsalo]

Closes none, but relates to nipreps/fmriprep@ea65196, nipreps/smriprep#446, and nipreps/nibabies#368. This will probably take effect at Nibabies 24.0.0 and fMRIPrep 24.0.2.

Changes proposed in this pull request

• Accept either no space entity (old/current versions for fMRIPrep and nibabies) or space-fsaverage (upcoming versions of fMRIPrep and nibabies) for surface spheres used in the anatomical workflow.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.28%. Comparing base (18e39a4) to head (e031dda).
Report is 58 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1217   +/-   ##
=======================================
  Coverage   84.28%   84.28%           
=======================================
  Files          54       54           
  Lines        7065     7065           
  Branches     1027     1027           
=======================================
  Hits         5955     5955           
  Misses        850      850           
  Partials      260      260           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.