add global debug flag

[zreigz]

Summary

Add global debug flag plural --debug or PLURAL_DEBUG_ENABLE=true plural ... to display some debug information

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Test Plan

Checklist

• [ ] If required, I have updated the Plural documentation accordingly.
• [ ] I have added tests to cover my changes.
• [ ] I have added a meaningful title and summary to convey the impact of this PR to a user.
• [ ] I have added relevant labels to this PR to help with categorization for release notes.