fix: connector's logger to format logs to match GCP format

[dario-commercetools]

Link to the Jira issue

[IM-579] https://commercetools.atlassian.net/browse/IM-579

Description and context

Just adding severity field to logs in order to match GCP format

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Brief description of the solution

How was it tested?

Instructions to build/test locally

Depends on another PR/branch?

• No
• Yes - dependant on PR/branch:

Checklist-

• I fixed|updated|added unit tests and integration tests for each feature (if applicable).
• No error nor warning in the console.
• I joined a screenshot of the app before and after the fix.

[upload the screenshot here]

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