Missing asynchronous context-free credential validation for DDF

[skateman]

There's no self.verify_credentials for the add provider form, also the already existing validation operates with DB models, so it's basically the same problem as in ManageIQ/manageiq-providers-kubernetes#387 but here for the default and the metrics endpoint.

@miq-bot assign @agrare
Parent issue: ManageIQ/manageiq#18818

[agrare]

Hey @skateman actually there is one it is just in InfraManager::ApiIntegrations. I had to update it to match what you have in #509 so I'll push the changes to your branch but verification and adding the provider works, going to close this