feat(pinia-orm): add option for custom pivotKey for relations with pivot

[CodeDredd]

🔗 Linked issue

Resolves: #1886

📚 Description

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 98.57143% with 1 line in your changes missing coverage. Please review.

Project coverage is 99.74%. Comparing base (220b093) to head (ef53c7b).
Report is 45 commits behind head on main.

Files with missing lines	Patch %	Lines
...rm/src/model/attributes/relations/MorphedByMany.ts	92.85%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1907      +/-   ##
==========================================
+ Coverage   99.72%   99.74%   +0.01%     
==========================================
  Files          90       90              
  Lines        6255     6287      +32     
  Branches      524      526       +2     
==========================================
+ Hits         6238     6271      +33     
  Misses         14       14              
+ Partials        3        2       -1     

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