Support multiple DNS server IPs

The host configuration format supports setting multiple DNS server IP addresses, see: https://git.glasklar.is/system-transparency/core/stboot/-/blob/d576dc81c6c6e0e49c689deb556fc791be2e5e5d/host/config.go#L207

However, stprov only supports populating that list with a single item (either using the -d option or a compiled-in default).