Add TreeFeatureSelectionTransform

[Mr-Geekman]

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• Did you read the contribution guide?
• Did you update the docs? We use Numpy format for all the methods and classes.
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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

Add TreeFeatureSelectionTransform that can select regressors.

Related Issue

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Closing issues

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[codecov-commenter]

Codecov Report

Merging #229 (8b94bc8) into master (288c5a9) will increase coverage by 0.21%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #229      +/-   ##
==========================================
+ Coverage   86.13%   86.35%   +0.21%     
==========================================
  Files          87       88       +1     
  Lines        4134     4199      +65     
==========================================
+ Hits         3561     3626      +65     
  Misses        573      573              

Impacted Files	Coverage Δ
etna/transforms/__init__.py	100.00% <100.00%> (ø)
etna/transforms/feature_importance.py	100.00% <100.00%> (ø)

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[julia-shenshina]

 shouldn't it be

Suggested change

	for column in df_transformed.columns.get_level_values("feature"):
	for column in set(df_transformed.columns.get_level_values("feature")):

?