This method returns a [hcldec.ObjectSpec](/guides/hcl/component-object-spec) of the data source output.
The object spec can be generated using the command [`mapstructure-to-hcl2`](https://github.com/hashicorp/packer/tree/master/cmd/mapstructure-to-hcl2)
The object spec can be generated using the command [`packer-sdc mapstructure-to-hcl2`](https://github.com/hashicorp/packer-plugin-sdk/tree/main/cmd/packer-sdc)
just like the configuration spec for the `ConfigSpec` method.
This method is used in `packer validate` command. Packer will use the spec to assign