Enable item search by refcode in the API

[ml-evs]

Previously the /items/<refcode> endpoint was not accessible through the Python API package (as it didn't exist at the time). This PR adds it.

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 36.90%. Comparing base (4336025) to head (1bb9679).
⚠️ Report is 15 commits behind head on main.

Files with missing lines	Patch %	Lines
src/datalab_api/__init__.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #64   +/-   ##
=======================================
  Coverage   36.90%   36.90%           
=======================================
  Files           4        4           
  Lines         485      485           
=======================================
  Hits          179      179           
  Misses        306      306           

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