Direct Recursive Strategy for prediction models

[authierj]

Checklist before merging this PR:

• Mentioned all issues that this PR fixes or addresses.
• Summarized the updates of this PR under Summary.
• Added an entry under Unreleased in the Changelog.

Fixes #1853.

Summary

In this PR, the direct recursive strategy for prediction models is added for regression models.

Other Information

TO DO:

• add to unit tests
• check that historical forecasts are working
• test with validation set

[codecov]

Codecov Report

❌ Patch coverage is 26.47059% with 50 lines in your changes missing coverage. Please review.
✅ Project coverage is 95.15%. Comparing base (e106771) to head (be596b7).

Files with missing lines	Patch %	Lines
darts/utils/multioutput.py	22.80%	44 Missing ⚠️
darts/models/forecasting/sklearn_model.py	45.45%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2941      +/-   ##
==========================================
- Coverage   95.52%   95.15%   -0.37%     
==========================================
  Files         146      146              
  Lines       15703    15766      +63     
==========================================
+ Hits        15000    15002       +2     
- Misses        703      764      +61     

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