Add concat resolver for OmegaConf

[egoriyaa]

Before submitting (must do checklist)

• Did you read the contribution guide?
• Did you update the docs? We use Numpy format for all the methods and classes.
• Did you write any new necessary tests?
• Did you update the CHANGELOG?

Proposed Changes

Closing issues

closes #603

[github-actions]

🚀 Deployed on https://deploy-preview-604--etna-docs.netlify.app

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.58%. Comparing base (0a0145e) to head (8d7b686).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #604   +/-   ##
=======================================
  Coverage   90.58%   90.58%           
=======================================
  Files         261      261           
  Lines       18050    18055    +5     
=======================================
+ Hits        16351    16356    +5     
  Misses       1699     1699           

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