Fix keys processing in BulkLoader

[roxblnfk]

🔍 What was changed

Now all the necessary keys (PK, FK) will be converted to scalar in same way. It respects DB's ValueInterface and Stringable

🤔 Why?

To use object IDs

📝 Checklist

• How was this tested:
  • Tested manually
  • Unit tests added

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.82%. Comparing base (c6d621b) to head (8f9d9d4).
⚠️ Report is 1 commits behind head on 2.x.

Additional details and impacted files

@@             Coverage Diff              @@
##                2.x     #558      +/-   ##
============================================
+ Coverage     91.80%   91.82%   +0.02%     
+ Complexity     1966     1963       -3     
============================================
  Files           131      131              
  Lines          5039     5054      +15     
============================================
+ Hits           4626     4641      +15     
  Misses          413      413              

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