qos: T6638: require interface state existence in verify conditional

[jestabro]

Change Summary

Add an optional strict requirement of interface state existence to verify_interface_exists: this has no effect unless keyword state_required=True, as needed in qos.py in case of a late arriving pppoe interface.

This restores the intended behavior of the conditional in qos.py:
ed63c9d#diff-c3e850625edc3ba2f44020fcda8640d00631b3c67bac756f258b919667b9d67eR306

The obvious question of how to approach re-applying qos.TrafficShaper.update when the interface is available (other than providing a warning to the user) is a separate issue which will need to be addressed in a subtask, if not obvious to those more familiar with the subsystem.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Code style update (formatting, renaming)
• Refactoring (no functional changes)
• Migration from an old Vyatta component to vyos-1x, please link to related PR inside obsoleted component
• Other (please describe):

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Checklist:

• I have read the CONTRIBUTING document
• I have linked this PR to one or more Phabricator Task(s)
• I have run the components SMOKETESTS if applicable
• My commit headlines contain a valid Task id
• My change requires a change to the documentation
• I have updated the documentation accordingly

[github-actions]

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No issues in PR Title / Commit Title

[c-po]

@Mergifyio backport sagitta circinus

[mergify]

backport sagitta circinus

✅ Backports have been created

• #3962 qos: T6638: require interface state existence in verify conditional (backport #3960) has been created for branch sagitta
• #3963 qos: T6638: require interface state existence in verify conditional (backport #3960) has been created for branch circinus