headlamp-plugin: Add command to validate artifacthub-pkg.yml

[illume]

Waiting 25 minutes per try to get the artifacthub validator to work by waiting for artifacthub to rescan is slow.

Describe the solution you'd like

A headlamp-plugin command that validates artifacthub-pkg.yml files

What users will benefit from this feature?

Being able to publish more easily.