Relationship Import for Parser Pipeline

[JSv4]

On reflection of existing components and workflows, it will be much easier (And probably more correct) to create parent child links as relationship objects. While we have a number of features to render and support this on the frontend, the backend was not actually importing or exporting relationships in our corpus exports. Updated base dict definitions and added importing utils and tests.

[codecov]

Codecov Report

Attention: Patch coverage is 97.75641% with 7 lines in your changes missing coverage. Please review.

Project coverage is 90.00%. Comparing base (1b44ce5) to head (3fc4fbc).
Report is 31 commits behind head on main.

Files with missing lines	Patch %	Lines
...ncontractserver/pipeline/parsers/docling_parser.py	87.50%	6 Missing ⚠️
opencontractserver/tasks/embeddings_task.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #276      +/-   ##
==========================================
+ Coverage   89.62%   90.00%   +0.38%     
==========================================
  Files         220      226       +6     
  Lines        7826     8076     +250     
==========================================
+ Hits         7014     7269     +255     
+ Misses        812      807       -5     

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