rf: Split premask workflow out of enhance_and_skullstrip_wf

[effigies]

Begins to address #1000. This will allow fmriprep to generate its pre-mask from its HMC boldref while enhancing and skull-stripping the sbref, if provided.

[codecov]

Codecov Report

❌ Patch coverage is 5.88235% with 16 lines in your changes missing coverage. Please review.
✅ Project coverage is 71.15%. Comparing base (338efa6) to head (9c3c22d).

Files with missing lines	Patch %	Lines
niworkflows/func/util.py	5.88%	16 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1001      +/-   ##
==========================================
- Coverage   71.20%   71.15%   -0.05%     
==========================================
  Files          89       89              
  Lines        8578     8585       +7     
  Branches      931      931              
==========================================
+ Hits         6108     6109       +1     
- Misses       2238     2244       +6     
  Partials      232      232              

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