Add self update command

[mostafa]

Add a command to update the GatewayD itself. This means two things:

1. Addition of a subcommand, like gatewayd self update, that updates the current binary to the latest version, which is the purpose of this ticket.
2. Hotswapping the current running instance of GatewayD with the updated version (#297)

Resources

• https://github.com/sanbornm/go-selfupdate