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 (b313cd9) to head (bdc917e).

❗ There is a different number of reports uploaded between BASE (b313cd9) and HEAD (bdc917e). Click for more details.

HEAD has 314 uploads less than BASE

Flag	BASE (b313cd9)	HEAD (bdc917e)
cpu	95	24
lightning	73	18
pytest	49	0
python3.9	24	6
lightning_fabric	12	0
python3.11	24	6
python3.10	12	3
python3.12.7	35	9
gpu	2	0
pytorch2.1	18	9
pytest-full	48	24
pytorch2.2.2	6	3
pytorch_lightning	12	6
pytorch2.3	6	3
pytorch2.5.1	12	6
pytorch2.4.1	6	3

Additional details and impacted files

@@            Coverage Diff            @@
##           master   #20596     +/-   ##
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- Coverage      88%      79%     -9%     
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  Files         267      264      -3     
  Lines       23369    23315     -54     
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- Hits        20478    18367   -2111     
- Misses       2891     4948   +2057