(app) Make Logging DEBUG mode lazy

[tchaton]

What does this PR do?

This PR improves framework debugging by implying how to enable logging debug mode.

Right now, there is 2 different ways to enable it.

LIGHTNING_DEBUG=1 lightning run app ...

import lightning as L

L.LightningApp(..., debug=True)

Fixes #<issue_number>

Does your PR introduce any breaking changes? If yes, please list them.

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[awaelchli]

If the user has regular logging, i.e. logging.getLogger(__name__) with debugging level instead of this special logger introduced here, this will still work as expected, right?

[tchaton]

If the user has regular logging, i.e. logging.getLogger(__name__) with debugging level instead of this special logger introduced here, this will still work as expected, right?

Yes.