Example (including tutorial) K8s charms should set charm-user

We should be able to set [charm-user](https://documentation.ubuntu.com/charmcraft/stable/reference/files/charmcraft-yaml-file/#charm-user) to `non-root` (or at least `sudoer`) for the Zero-to-Hero charm and any example charms. We should set `uid` and `gid` for the container as well, I believe.

I'm not sure if this would be a "needs to wait until Charmcraft 5" item, but I wonder if we also should have the `kubernetes` profile have this set to `non-root` with a comment (and link to the docs) that it can be changed if the charm has to run as root or have sudo for some reason. Defaulting to the lower privileges is generally a good idea.