Extend CLI for estimation of the number of folds

[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

Closing issues

closes #1237

[github-actions]

🚀 Deployed on https://deploy-preview-1284--etna-docs.netlify.app

[codecov-commenter]

Codecov Report

Merging #1284 (414ac71) into master (06cc015) will decrease coverage by 0.17%.
The diff coverage is 92.68%.

❗ 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    #1284      +/-   ##
==========================================
- Coverage   88.06%   87.89%   -0.17%     
==========================================
  Files         187      176      -11     
  Lines       10825    10544     -281     
==========================================
- Hits         9533     9268     -265     
+ Misses       1292     1276      -16     

Impacted Files	Coverage Δ
etna/commands/forecast_command.py	94.80% <89.47%> (-2.03%)	⬇️
etna/commands/backtest_command.py	95.91% <94.11%> (-1.15%)	⬇️
etna/commands/utils.py	100.00% <100.00%> (ø)

... and 12 files with indirect coverage changes

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