add RMSE metric & rmse functional metric

[smetam]

Before submitting (must do checklist)

• [ x] Did you read the contribution guide?
• [ x] Did you update the docs? We use Numpy format for all the methods and classes.
• [ x] Did you write any new necessary tests?
• [ x] Did you update the CHANGELOG?

Proposed Changes

Closing issues

closes #1031

[codecov-commenter]

Codecov Report

Merging #1051 (61ed089) into master (92e1ba4) will increase coverage by 0.11%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1051      +/-   ##
==========================================
+ Coverage   86.13%   86.25%   +0.11%     
==========================================
  Files         163      163              
  Lines        8629     8641      +12     
==========================================
+ Hits         7433     7453      +20     
+ Misses       1196     1188       -8     

Impacted Files	Coverage Δ
etna/metrics/__init__.py	100.00% <100.00%> (ø)
etna/metrics/functional_metrics.py	86.95% <100.00%> (+1.95%)	⬆️
etna/metrics/metrics.py	100.00% <100.00%> (ø)
etna/auto/runner/utils.py	50.00% <0.00%> (-50.00%)	⬇️
etna/libs/tsfresh/significance_tests.py	46.37% <0.00%> (+14.49%)	⬆️

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[alex-hse-repository]

May you also add a ling to the current PR in the format "([#<pr number>](link))". See the raw view of CHANGELOG.md file for the examples

[smetam]

Done

[alex-hse-repository]

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