feat: DIA-1061: Add START_TRAINING to fit in models

[makseq]

This PR adds START_TRAINING webhook event to fit() methods. See https://github.com/HumanSignal/label-studio/pull/5761 for details.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 22.22222% with 7 lines in your changes are missing coverage. Please review.

Project coverage is 46.50%. Comparing base (71a50c0) to head (c0f6e16).

Files	Patch %	Lines
...tudio_ml/examples/sklearn_text_classifier/model.py	0.00%	5 Missing :warning:
label_studio_ml/examples/bert_classifier/model.py	0.00%	2 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #529      +/-   ##
==========================================
- Coverage   51.17%   46.50%   -4.67%     
==========================================
  Files          38       17      -21     
  Lines        2474     1103    -1371     
==========================================
- Hits         1266      513     -753     
+ Misses       1208      590     -618     

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[makseq]

/git merge master