Allow a custom parser class when using LightningCLI

[mauvilsa]

What does this PR do?

Recently jsonargparse added new possibilities by customizing the argument parser class, for example:

• Question: is it possible to make CLI replace parameter dashes with underscores? omni-us/jsonargparse#301
• CLI: can bool parameters become --flag instead of --flag true? omni-us/jsonargparse#355

However, currently LightningCLI doesn't allow changing the parser class to use. This pull request makes that possible.

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📚 Documentation preview 📚: https://pytorch-lightning--20596.org.readthedocs.build/en/20596/

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79%. Comparing base (a3314d4) to head (bbf8c73).
Report is 112 commits behind head on master.

Additional details and impacted files

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##           master   #20596   +/-   ##
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  Coverage      79%      79%           
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  Files         264      264           
  Lines       23316    23317    +1     
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+ Hits        18326    18372   +46     
+ Misses       4990     4945   -45