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

Before submitting

• [ ] Was this discussed/approved via a Github issue? (no need for typos and docs improvements)
• [x] Did you read the contributor guideline, Pull Request section?
• [x] Did you make sure your PR does only one thing, instead of bundling different changes together?
• [n/a] Did you make sure to update the documentation with your changes?
• [x] Did you write any new necessary tests? [not needed for typos/docs]
• [x] Did you verify new and existing tests pass locally with your changes?
• [ ] If you made a notable change (that affects users), did you update the CHANGELOG?

PR review

• [x] Is this pull request ready for review? (if not, please submit in draft mode)

Anyone in the community is free to review the PR once the tests have passed. If we didn't discuss your PR in Github issues there's a high chance it will not be merged.

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Make sure you had fun coding 🙃

[codecov[bot]]

Codecov Report

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

Additional details and impacted files

```diff @@ 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!