Use urfave/cli v2 as argparser

[Noroth]

Use https://github.com/spf13/cobra as a better arg parser instead of the flag package.

Use urfave/cli v2 as arg parser similar to its use in reg-room-service/cmd/main.go.

[Jumpy-Squirrel]

Cobra might be overkill. I can sort of see us using pflag, which cobra is based on, but when we go to containers (soon?) some of the flags will go away:

• migrate database would become a pre-deployment job container, or else we must do complicated gymnastics to prevent starting two instances of our service at the same time which both try to migrate the database, leaving it in unusable state
  (edit: this is a separate issue and not part of this issue. For now, we stick with migrate-database as is)
• log style selection should default to json logging, with the switch to human readable a field in the config.yaml. It's not a problem if the service starts with json logging on dev machines and switches after the first line
• the path to the config file should be fixed, possibly allowing another override via environment variable
• (did I forget any cmd line flags?)

Edit: I like what the room-service does, it's so clear and simple, we should just stick with it.

[Jumpy-Squirrel]

New target solution: github.com/urfave/cli (v2) instead of cobra.

Compare https://github.com/eurofurence/reg-room-service/blob/main/cmd/reg-room-service/main.go this is extremely clear and very easy to understand, and allows setting everything via env if desired.