Forecast decomposition for CatBoost models

[brsnw250]

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 #1135

[codecov-commenter]

Codecov Report

Merging #1148 (0e82c81) into master (3d76478) will decrease coverage by 0.20%.
The diff coverage is 100.00%.

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@@            Coverage Diff             @@
##           master    #1148      +/-   ##
==========================================
- Coverage   87.00%   86.81%   -0.20%     
==========================================
  Files         177      177              
  Lines        9847     9856       +9     
==========================================
- Hits         8567     8556      -11     
- Misses       1280     1300      +20     

Impacted Files	Coverage Δ
etna/models/catboost.py	100.00% <100.00%> (ø)
etna/auto/runner/utils.py	50.00% <0.00%> (-50.00%)	⬇️
etna/libs/tsfresh/significance_tests.py	31.88% <0.00%> (-15.95%)	⬇️
etna/core/mixins.py	95.23% <0.00%> (-3.41%)	⬇️
etna/libs/tsfresh/relevance.py	76.19% <0.00%> (-1.91%)	⬇️
etna/models/nn/utils.py	83.94% <0.00%> (-0.12%)	⬇️
etna/models/utils.py	100.00% <0.00%> (ø)
etna/models/nn/mlp.py	100.00% <0.00%> (ø)
etna/models/nn/rnn.py	100.00% <0.00%> (ø)

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[github-actions]

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