Using job class instead of hardcodced Job

[ilongin]

Using job_class private property instead of Job to avoid issues in SaaS where we have extended class with additional fields.

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[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 86.78%. Comparing base (16ed477) to head (ba50c45).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/datachain/data_storage/metastore.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #391   +/-   ##
=======================================
  Coverage   86.78%   86.78%           
=======================================
  Files          92       92           
  Lines       10121    10121           
  Branches     2054     2054           
=======================================
  Hits         8784     8784           
  Misses        988      988           
  Partials      349      349           

Flag	Coverage Δ
datachain	86.72% <0.00%> (ø)

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