feat: Add flag to fallback to Estimated* metadata or a passed value for TotalReadoutTime

[effigies]

This flag provides two additional ways to retrieve TotalReadoutTime. If EstimatedTotalReadoutTime or EstimatedEffectiveEchoSpacing are defined, the user can pass --fallback-total-readout-time estimated to direct fMRIPrep to use these.

This could be turned on by default, but I don't think this is prudent. The reason that dcm2niix does not simply say TotalReadoutTime or EffectiveEchoSpacing is not to misrepresent the confidence that these are correct. Previously, users had to actively remove the Estimated to accept the estimates, and I don't think we want to eliminate the conscious decision. However, we can make it so that users do not need to modify their datasets to accept them.

Closes #3009.

[codecov]

Codecov Report

Attention: Patch coverage is 46.15385% with 7 lines in your changes missing coverage. Please review.

Project coverage is 72.25%. Comparing base (64e27fb) to head (986528d).

Files with missing lines	Patch %	Lines
fmriprep/cli/parser.py	33.33%	6 Missing ⚠️
fmriprep/workflows/base.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3423      +/-   ##
==========================================
- Coverage   72.33%   72.25%   -0.08%     
==========================================
  Files          57       57              
  Lines        4269     4282      +13     
  Branches      545      546       +1     
==========================================
+ Hits         3088     3094       +6     
- Misses       1065     1072       +7     
  Partials      116      116              

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