Add optional parameter to pass the IP address. Avoids querying ip.hetzner.com

[FlexApex]

When updating the IP from an ppp-up script, the local IP is already known and doesn't have to be queried.
This PR adds the optional parameter -i <ip> and, if passed, doesn't query ip.hetzner.com for the IP.
Saves resources and useful for multi-WAN setups, where you want to update for a different interface than the current route.

Example use:

#!/bin/bash

HETZNER_AUTH_API_TOKEN="..."
logger -t hetznerDNS "Updating Hetzner DNS: dyndns.example.com to new IP $4"
/usr/local/bin/hetzner-dns-update.sh -Z example.com -n dyndns -i $4