Series decomposition transform with FFT

[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 #425

[github-actions]

🚀 Deployed on https://deploy-preview-430--etna-docs.netlify.app

[codecov]

Codecov Report

Attention: Patch coverage is 90.90909% with 8 lines in your changes missing coverage. Please review.

Project coverage is 86.76%. Comparing base (4a8bbb5) to head (c266594).
Report is 4 commits behind head on master.

Files	Patch %	Lines
etna/transforms/decomposition/dft_based.py	90.69%	8 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           master     #430       +/-   ##
===========================================
+ Coverage    9.61%   86.76%   +77.14%     
===========================================
  Files         226      228        +2     
  Lines       15594    15854      +260     
===========================================
+ Hits         1500    13755    +12255     
+ Misses      14094     2099    -11995     

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