[Shipping Labels] Add entry point to new shipping label flow

[rachelmcr]

Conditions to show new shipping label flow:

• Feature flag for the new flow is enabled
• Shipping extension (with minimum version) is active

Tasks:

• [x] Add new feature flag
• [x] Add check for feature flag and extension
• [ ] Navigate to the correct shipping label screen based on the check

[dangermattic]

Thanks for reporting! 👍