Add `opennebula_marketplace_app` resource and data source

[frousselet]

Description

Along side of opennebula_marketplace resource and data source (#414), we should add an opennebula_marketplace_app resource and data source to create apps within the created marketplace.

We might be able to retrieve data source by name or id.

New or affected resources and data sources

opennebula_marketplace

Potential terraform configuration

resource "opennebula_marketplace_app" "my_app" {
  name = "My App"
  description = "This is my amazing app !"
  version = "0.2"
  image = opennebula_image.my_image.id
  marketplace = opennebula_marketplace.my_marketplace.id
  vm_template = "..."
  app_template = "..."
}

References

414

[treywelsh]

List of attributes: https://docs.opennebula.io/6.6/marketplace/appliances/marketapps.html#marketplace-appliance-attributes