Add 5 legalbench tasks (the 5 existing in HELM)

[perlitz]

The LegalBench project is an ongoing open science effort to collaboratively curate tasks for evaluating legal reasoning in English large language models (LLMs). The benchmark currently consists of 162 tasks gathered from 40 contributors.

https://github.com/HazyResearch/legalbench/tree/main

Here, although adding only the 5 tasks appearing in HELM, we are also laying the foundations for adding many more of them.

[codecov]

Codecov Report

Attention: Patch coverage is 75.00000% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 92.11%. Comparing base (5783d76) to head (71b4b42).
Report is 52 commits behind head on main.

Files	Patch %	Lines
src/unitxt/string_operators.py	75.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #827      +/-   ##
==========================================
+ Coverage   91.48%   92.11%   +0.63%     
==========================================
  Files         100      104       +4     
  Lines       10169    10695     +526     
==========================================
+ Hits         9303     9852     +549     
+ Misses        866      843      -23     

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