Add support of quantiles in backtest

[martins0n]

IMPORTANT: Please do not create a Pull Request without creating an issue first.

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?

Type of Change

• Examples / docs / tutorials / contributors update
• Bug fix (non-breaking change which fixes an issue)
• Improvement (non-breaking change which improves an existing feature)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Proposed Changes

Related Issue

Closing issues

[github-actions]

🚀 Deployed on https://deploy-preview-652--etna-docs.netlify.app

[codecov-commenter]

Codecov Report

Merging #652 (596f4b5) into master (0f7e517) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #652   +/-   ##
=======================================
  Coverage   84.44%   84.45%           
=======================================
  Files         119      119           
  Lines        6320     6322    +2     
=======================================
+ Hits         5337     5339    +2     
  Misses        983      983           

Impacted Files	Coverage Δ
etna/pipeline/base.py	94.46% <100.00%> (+0.04%)	⬆️

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