prometheus-node-exporter-lua: Make listen IP address configurable

Maintainer: @champtar

Description:

The init script only allows the exporter to be configured to bind to an ip address from an openwrt managed interface, this excludes interfaces configured externally, aka "unmanaged" protocol in the network config. For mesh network solutions like zerotier, ip addresses are assigned dynamically by a controller. Also an interface can have multiple ipv6 addresses configured, and some hosts might only be able to reach the node exporter from a specific address.

I propose to add another option called listen_addrs, being a list of addresses to bind to, to simplify the code, ipv6 addresses will have to be wrapped in brackets unless there's a simple way to check for valid ipv6 addresses. If this list is given, the option listen_interface should be ignored.