Create EventTransform

[egoriyaa]

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 #14

[codecov]

Codecov Report

Patch coverage: 98.38% and project coverage change: +0.04% 🎉

Comparison is base (adfd18c) 89.23% compared to head (c398c0f) 89.27%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #78      +/-   ##
==========================================
+ Coverage   89.23%   89.27%   +0.04%     
==========================================
  Files         205      206       +1     
  Lines       12807    12869      +62     
==========================================
+ Hits        11428    11489      +61     
- Misses       1379     1380       +1     

Files Changed	Coverage Δ
etna/transforms/timestamp/event.py	98.33% <98.33%> (ø)
etna/transforms/__init__.py	100.00% <100.00%> (ø)
etna/transforms/timestamp/__init__.py	100.00% <100.00%> (ø)

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[github-actions]

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