[WIP] Add tslearn.foundation module for re-using pre-trained time series models (typically from HF)

[rtavenar]

Introduces ZeroShotForecaster, LinearProbeForecaster, LinearProbeClassifier and the underlying TimeSeriesFoundationEmbedder, letting pre-trained models (e.g. Chronos-2) be used behind the usual tslearn estimator API, either zero-shot or with a linear head fitted on frozen representations.

[codecov]

Codecov Report

❌ Patch coverage is 99.83333% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 95.90%. Comparing base (4cf2840) to head (3817518).
⚠️ Report is 11 commits behind head on main.

Files with missing lines	Patch %	Lines
tslearn/foundation/_forecasting.py	99.23%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #699      +/-   ##
==========================================
+ Coverage   95.14%   95.90%   +0.75%     
==========================================
  Files          83       87       +4     
  Lines        7814     9214    +1400     
==========================================
+ Hits         7435     8837    +1402     
+ Misses        379      377       -2     

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