PB-381: make the log level configurable

[little-dude]

This is based on #242

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References

https://xainag.atlassian.net/browse/PB-381

Summary

Add a new [logging] section to the configuration file. For now, this
section only provides a level key which can be one of the log levels
supported by default by the Python standard library: notset, debug,
info, warning, error, critical.

In the future, this [logging] section could be used to provide more
knobs for logging customatization: enabling colors for the terminal
handler, specifying a path for logging into a file, etc.

References:

xainag.atlassian.net/browse/PB-381

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Reviewer checklist

Reviewer agreement:

• Reviewers assign themselves at the start of the review.
• Reviewers do not commit or merge the merge request.
• Reviewers have to check and mark items in the checklist.

Merge request checklist

• Conforms to the merge request title naming XP-XXX <a description in imperative form>.
• Each commit conforms to the naming convention XP-XXX <a description in imperative form>.
• Linked the ticket in the merge request title or the references section.
• Added an informative merge request summary.

Code checklist

• Conforms to the branch naming XP-XXX-<a_small_stub>.
• Passed scope checks.
• Added or updated tests if needed.
• Added or updated code documentation if needed.
• Conforms to Google docstring style.
• Conforms to XAIN structlog style.