Add regressors property to TSDataset

[Mr-Geekman]

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• Was this discussed/approved via a Github issue? (no need for typos and docs improvements)
• Did you read the contribution guide?
• Did you check the code style? make lint (poetry install -E style).
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• Did you write any new necessary tests?
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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 regressors property to TSDataset which returns dict where keys are segments and values are regressor columns for these segments.

Rewrite make_future to use this new property.

Related Issue

#75.

PR review

Anyone in the community is free to review the PR once the tests have passed.
If we didn't discuss your PR in Github issues there's a high chance it will not be merged.

Closing issues

Closes #75.

[codecov-commenter]

Codecov Report

Merging #82 (52fb8e5) into master (9d4f55f) will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #82      +/-   ##
==========================================
+ Coverage   88.03%   88.08%   +0.04%     
==========================================
  Files          55       55              
  Lines        2541     2551      +10     
==========================================
+ Hits         2237     2247      +10     
  Misses        304      304              

Impacted Files	Coverage Δ
etna/datasets/tsdataset.py	87.43% <100.00%> (+0.66%)	⬆️

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[julia-shenshina]

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