Feat/ Adding sample weight

[AntonRagot]

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 #1175.

Summary

• Adding the option to choose between three possibilities of predefined weights
• fit now has the option to support sample_weight, which can be a string or a TimeSeries

[codecov]

Codecov Report

Attention: Patch coverage is 78.57143% with 9 lines in your changes are missing coverage. Please review.

Project coverage is 93.67%. Comparing base (e2085ae) to head (c6395e8).

Files	Patch %	Lines
darts/utils/timeseries_generation.py	22.22%	7 Missing ⚠️
darts/models/forecasting/regression_model.py	90.90%	1 Missing ⚠️
darts/utils/data/tabularization.py	95.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2362      +/-   ##
==========================================
- Coverage   93.73%   93.67%   -0.06%     
==========================================
  Files         138      138              
  Lines       14309    14330      +21     
==========================================
+ Hits        13412    13424      +12     
- Misses        897      906       +9     

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