Added table serializers operators and add Wikitq table question answering dataset

[csrajmohan]

Table serializers convert a given table into a flat sequence with special symbols. This data preparation step is essential for running tabular data tasks with LLMs. As part of this PR,

• Created base class for Table serializers and added 2 main table serializers(Indexed Row major, Markdown) as new Unitxt operators.
• Added a new task card for WikiTableQuestions(wikitq) dataset and validated the serializer operators with table input.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (099c601) 93.93% compared to head (31784ae) 93.93%.
Report is 2 commits behind head on main.

❗ Current head 31784ae differs from pull request most recent head 37b24a2. Consider uploading reports for the commit 37b24a2 to get more accurate results

Additional details and impacted files

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  Coverage   93.93%   93.93%           
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  Files         207      207           
  Lines        8009     8009           
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  Hits         7523     7523           
  Misses        486      486           

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[elronbandel]

Well done great contribution!