OpenVPN has suppport for (multivalued) DHCP option DOMAIN-SEARCH (119), which is ignored by nm-opevpn.
Core NetworkManager doesn't have an option for them either.
However, in practical terms it appears to make sense to just append them to the domains list.
So handle DOMAIN-SEARCH options mostly similar to DOMAIN ones. Since it's not unlikely that some servers may use multiple DOMAIN and DOMAIN-SEARCH values, accumulate the latter ones in a separate array first and then append it to the tail of the domains array.