[ENH] Trend as an Effect

[felipeangelimvieira]

This PR further changes Effect API so that Trend models also use BaseEffect and same conventions.

• fit receives y, X and scale. Returns None.
• transform receives X and fh, where fh must be an index. It eturns the data that will be used in predict.
• predict receives data (with the output of transform) and predicted_effects, which can be a dict with previous predicted effects (including trend).

Trend classes are now within prophetverse.effects.trend.

Next PR will take care of documentation updates

[codecov]

Codecov Report

Attention: Patch coverage is 98.11321% with 4 lines in your changes missing coverage. Please review.

Project coverage is 94.64%. Comparing base (591099e) to head (cc948e5).

Files	Patch %	Lines
src/prophetverse/sktime/base.py	93.18%	3 Missing ⚠️
src/prophetverse/effects/base.py	96.29%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #95      +/-   ##
==========================================
+ Coverage   93.69%   94.64%   +0.94%     
==========================================
  Files          26       26              
  Lines        1190     1177      -13     
==========================================
- Hits         1115     1114       -1     
+ Misses         75       63      -12     

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