Fix use of jsonargparse avoiding reliance on non-public internal logic

[mauvilsa]

What does this PR do?

Fixes issues identified in #1564. More generally, this pull request changes flash_cli.py and lightning_cli.py such that they only use jsonargparse's public API.

closes #1564

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• [n/a] Did you make sure to update the documentation with your changes?
• Did you write any new necessary tests? [not needed for typos/docs]
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[codecov]

Codecov Report

Merging #1620 (ed74d4c) into master (30bf0f5) will decrease coverage by 0%.
The diff coverage is 50%.

Additional details and impacted files

@@          Coverage Diff           @@
##           master   #1620   +/-   ##
======================================
- Coverage      86%     85%   -0%     
======================================
  Files         291     291           
  Lines       12876   12867    -9     
======================================
- Hits        11013   10990   -23     
- Misses       1863    1877   +14     

[Borda]

@mauvilsa awesome, thank you!