Add STAC output as option for `planet orders request`

[cholmes]

Right now there's no way in the CLI to request stac output, unless you hand craft your entire request. We should add a --metadata stac option to be able to include it in planet orders request.

[cholmes]

I'm leaning towards just making stac the default, with a --stac false for people who explicitly don't want STAC. But afaik it's very little overhead, and it's a small file. But enables a useful set of tools.