sg: use 1pass instead of dev-private

Created by: jhchabran

Drop dev-private entirely in favour of 1password.

Fixes https://github.com/sourcegraph/sourcegraph/issues/29640

• Find a better name for sg secret download-config
• Provide a better error when running enteprise code without having ran the command
• Maybe plug the requires_dev_private: true field to call sg secret download-config's code automatically ?

Test plan

• deleted ../dev-private
• ran sg secret download-config (with 1pass from sg setup configured)
• ran sg start and got it working, with the right site-config and license.