Teach catboost to work with encoders

[Mr-Geekman]

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

1. Fix bug encoders and new str/category values.
2. Make casting for category float types.

Closing issues

Closes #938.

[codecov-commenter]

Codecov Report

Merging #957 (4a0d286) into master (1f1f5b6) will increase coverage by 0.16%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #957      +/-   ##
==========================================
+ Coverage   85.34%   85.50%   +0.16%     
==========================================
  Files         148      148              
  Lines        7951     7963      +12     
==========================================
+ Hits         6786     6809      +23     
+ Misses       1165     1154      -11     

Impacted Files	Coverage Δ
etna/models/catboost.py	100.00% <100.00%> (ø)
etna/transforms/encoders/categorical.py	100.00% <100.00%> (ø)
etna/models/tbats.py	94.64% <0.00%> (+1.78%)	⬆️
etna/models/nn/tft.py	100.00% <0.00%> (+1.88%)	⬆️
etna/models/nn/deepar.py	100.00% <0.00%> (+2.27%)	⬆️
etna/auto/utils.py	94.73% <0.00%> (+21.05%)	⬆️
etna/auto/runner/utils.py	100.00% <0.00%> (+50.00%)	⬆️

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[github-actions]

🚀 Deployed on https://deploy-preview-957--etna-docs.netlify.app

[martins0n]

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