👌 Remove hard-coding of completion and duration need fields

[chrisjsewell]

As discussed in #1122 (comment),
These fields are used by needgantt but their name can be configured by needs_duration_option and needs_completion_option.

So here we remove the hard-coding and dynamically use these configuration options to set what option/key names are added.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.81%. Comparing base (3719cbd) to head (d7d82a0).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1127      +/-   ##
==========================================
- Coverage   85.81%   85.81%   -0.01%     
==========================================
  Files          56       56              
  Lines        6508     6505       -3     
==========================================
- Hits         5585     5582       -3     
  Misses        923      923              

Flag	Coverage Δ
pytests	85.81% <100.00%> (-0.01%)	⬇️

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