Add DeseasonalityTransform

[ostreech1997]

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 #680.

[github-actions]

🚀 Deployed on https://deploy-preview-1307--etna-docs.netlify.app

[codecov-commenter]

Codecov Report

Merging #1307 (bbede11) into master (991c648) will increase coverage by 0.01%.
The diff coverage is 91.13%.

❗ Your organization is not using the GitHub App Integration. As a result you may experience degraded service beginning May 15th. Please install the Github App Integration for your organization. Read more.

@@            Coverage Diff             @@
##           master    #1307      +/-   ##
==========================================
+ Coverage   88.65%   88.66%   +0.01%     
==========================================
  Files         196      197       +1     
  Lines       12048    12127      +79     
==========================================
+ Hits        10681    10753      +72     
- Misses       1367     1374       +7     

Impacted Files	Coverage Δ
etna/transforms/decomposition/deseasonal.py	90.90% <90.90%> (ø)
etna/transforms/__init__.py	100.00% <100.00%> (ø)
etna/transforms/decomposition/__init__.py	100.00% <100.00%> (ø)

📣 We’re building smart automated test selection to slash your CI/CD build times. Learn more

[Mr-Geekman]

We can do this by using enum. Value checking will be made there.