Implement SumTranform

[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

Look at #1013

Closing issues

Closes #1013

[github-actions]

🚀 Deployed on https://deploy-preview-1021--etna-docs.netlify.app

[codecov-commenter]

Codecov Report

Merging #1021 (a32c680) into master (cfbfb01) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1021      +/-   ##
==========================================
- Coverage   86.11%   86.11%   -0.01%     
==========================================
  Files         162      162              
  Lines        8613     8627      +14     
==========================================
+ Hits         7417     7429      +12     
- Misses       1196     1198       +2     

Impacted Files	Coverage Δ
etna/transforms/__init__.py	100.00% <100.00%> (ø)
etna/transforms/math/__init__.py	100.00% <100.00%> (ø)
etna/transforms/math/statistics.py	99.39% <100.00%> (+0.04%)	⬆️
etna/auto/runner/utils.py	50.00% <0.00%> (-50.00%)	⬇️

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[alex-hse-repository]

Looks good 👍
Add one test case and update the Changelog