multiple fixes for push/rollback demo

[JimOverholt]

Pull Request summary:

• add UX2Config model validator which inserts internal parcel type_ field from TransformedParcel header information
• fix Default[T] None value
• fix URLParcel type field name
• skip pushing unsupported parcels

Checklist:

• Make sure to run pre-commit before committing changes
• Make sure all checks have passed
• PR description is clear and comprehensive
• Mentioned the issue that this PR solves (if applicable)
• Make sure you test the changes

[JimOverholt]

This issue was migrated from an old repository: CiscoDevNet/catalystwan#538