Add into MAE ability to work with missing values

[d-a-bunin]

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

See #519.

Closing issues

Closes #519.

[github-actions]

🚀 Deployed on https://deploy-preview-523--etna-docs.netlify.app

[codecov]

Codecov Report

Attention: Patch coverage is 91.66667% with 1 line in your changes missing coverage. Please review.

Project coverage is 90.32%. Comparing base (82c1be2) to head (adb05c9).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
etna/metrics/functional_metrics.py	88.88%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #523      +/-   ##
==========================================
- Coverage   90.32%   90.32%   -0.01%     
==========================================
  Files         256      256              
  Lines       17214    17221       +7     
==========================================
+ Hits        15549    15555       +6     
- Misses       1665     1666       +1     

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