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 64.53%. Comparing base (d991644) to head (0032f04).

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

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1001      +/-   ##
==========================================
- Coverage   67.13%   64.53%   -2.61%     
==========================================
  Files          54       54              
  Lines        6850     6857       +7     
  Branches      852      852              
==========================================
- Hits         4599     4425     -174     
- Misses       2036     2241     +205     
+ Partials      215      191      -24     

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