[cmd] Implement rootless deprecation messages

I have added a system for showing fatal and non-fatal deprecation warnings. It's configurable by command and environment.

If we merge this PR, running a rootless image with any OPA command other than `opa run` will result in a fatal error and exit code 1.

It's possible for users to continue to use the image by unsetting: OPA_DOCKER_IMAGE_TAG=rootless.

`opa run` will show the message, but it's not fatal for this command. This is intended to avoid production disruption.

Signed-off-by: Charlie Egan <charlie@styra.com>