Add a CLI option to control logging level

[ackintosh]

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Description of the PR

closes #989

This PR makes logging level controllable with the property as below.

$ java -Dlog.level=error ....

[ackintosh]

#1000 👀

[ackintosh]

This PR is not targeting a particular programming language.
cc @OpenAPITools/generator-core-team @OpenAPITools/openapi-generator-collaborators

[wing328]

@ackintosh thanks for the enhancement, which definitely makes it easier to debug OpenAPI Generator (in addition to providing a configuration file)

Instead of using log_level, may I suggest naming the system property as log.level instead as it seems to be more common to use dots instead of underscores in system property? (but I couldn't find any official style guide for naming system properties)

Let's see if others have comments on this as well.

[jimschubert]

I agree that it's probably better as log.level so other logger implementations that we might not be wrapping could pick this up.

[ackintosh]

Thanks for the suggestion. It looks better to me as well. ✨

[ackintosh]

I've updated the property name. Please have a look when you have time.

[wing328]

LGTM