Add ability to download and install the `op` CLI tool

[joshuadavidthomas]

It'd be nice to either have a management command to install the CLI tool or even download it automatically if not found. I'm thinking of how django-tailwind-cli auto-downloads the Tailwind CSS CLI if the command is not available.

[joshuadavidthomas]

Hm, the more I think about this, the more I come down on the "not my problem" side. It would be nice to have, but I'm not sure I want to devote the time to developing this feature. We can just say the op CLI tool must be installed in order to use this field and be done with it.

Leaving this open in case anyone wants to pick it up and do it themselves, but I'm putting this on the backburner personally.