Preserve immutable STIX fields during workflow updates

[adpare]

Saving a relationship updates the workflow state of its source and target through PUTs. Those requests omitted a couple of stored STIX fields, and the backend interpreted their absence as an attempt to alter an immutable revision.

Any new relationship created was causing this issue. This PR addresses that by persisting the x_mitre_modief_by_ref and object_marking_refs values.

Related to PR open on the workbench frontend

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.12%. Comparing base (65ebb4e) to head (b12204c).
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Additional details and impacted files

@@           Coverage Diff           @@
##             beta     #497   +/-   ##
=======================================
  Coverage   81.12%   81.12%           
=======================================
  Files         245      245           
  Lines       39356    39366   +10     
  Branches     4820     4822    +2     
=======================================
+ Hits        31926    31936   +10     
  Misses       7342     7342           
  Partials       88       88           

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