Plan/Apply `-json` support missing about terraform-exec

[till]

As a workaround:

        ctx := context.Background()
        outfile := "./mystate.state"
	tfexec.Plan(context, tfexec.Out(outfile))

	tfjson, _ := txexec.ShowPlanFile(context, outfile)

	b, _ := json.Marshal(tfjson)

	jsonOutfile := "./mystate.json"
	os.WriteFile(jsonOutfile, b, 0666)

[bendbennett]

We have a use case for being able to stream JSON with the -json flag in Ability to test whether warnings are raised during test steps.

The intention is to modify the acceptance testing framework so that the -json flag can be used with terraform-exec to allow parsing of the streamed JSON in order to be able to verify that warning diagnostics are being emitted in specific tests.