Set log to debug on unknown facet entry

[wslulciuc]

Problem

The unknown_facet was added to help in the early stages of our spark integration, but the facet is also logged which can pollute logs.

Solution

Set log to debug on unknown facet entry.

Checklist

• You've signed-off your work
• Your pull request title follows our guidelines
• Your changes are accompanied by tests (if relevant)
• Your change contains a small diff and is self-contained
• You've updated any relevant documentation (if relevant)
• You've updated the CHANGELOG.md with details about your change under the "Unreleased" section (if relevant, depending on the change, this may not be necessary)
• You've versioned the core OpenLineage model or facets according to SchemaVer (if relevant)