Expose --tag command to delete all resources that have a tag.

**Is your feature request related to a problem? Please describe.**
A convenient way to delete all resources with a specific tag.

**Describe the solution you'd like**
`az resource delete --tag # deletes all resources that have a specific tag.`
`az vm delete --tag # deletes all VMs that have a specific tag.`
`az network vnet --tag # deletes all vnets that have a specific tag.`

Follow up from #4897

`az vm create ... --tags` applies tags to every resource it creates. Using this in conjunction with `az resource delete --tag` will not only delete the VM but every resource that was created to deploy the VM.

**Describe alternatives you've considered**

**Additional context**
- `az vm create's --tags` does not document that the tags are applied to every resource created via VM create. The parameter help should be updated.
- @tjprescott suggests implementing a delete_command() type to support tag-based deletion for any command.
- It might be good to restrict tag-based deletion to a resource group scope. So that resources with the same tag in another resource-group are not accidentally deleted.