feat(models): add job metrics in workflow model

[volodymyrss]

In order to store job metrics in Reana DB, we need to introduce a field. This is a first attempt.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.81%. Comparing base (f0da4c1) to head (1cbbb26).
⚠️ Report is 13 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #245      +/-   ##
==========================================
+ Coverage   72.79%   72.81%   +0.02%     
==========================================
  Files           7        7              
  Lines         985      986       +1     
==========================================
+ Hits          717      718       +1     
  Misses        268      268              

Files with missing lines	Coverage Δ
reana_db/models.py	90.60% <100.00%> (+0.01%)	⬆️

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