[v2] Accept `Inventory` flags in skaffold cmds

Add three flags `inventory-id`, `inventory-name` and `inventory-namespace` that should satisfy the following conditions

1. Set to global flags. We decide not to tie the flags to any specific skaffold pipeline steps. For example, inventory is a skaffold(kpt) deploy step arguments but in certain situations users expect to configure the inventory in render process. Since the operation is configuration related, skaffold supposes to support it even without deployment actions.
2. Set the flags to be hidden. These flags are requested for a specific use case (e.g. cloud deploy). We can expose the flags if more users request this feature
3. Unlike other flags (which overrides the equivalent values in skaffold.yaml), these flags are subservient to skaffold.yaml if `deploy.kpt.inventory` is given.
4. When used and the `deploy.kpt` (default to kubectl) in `skaffold.yaml` is specified, this inventory is always written into the /Kptfile.