build_openvpn_connection() — D-Bus settings construction

[cachebag]

part of #288. depends on OpenVpnConfig.

changes

add build_openvpn_connection(config, opts) in api/builders/openvpn_builder.rs. constructs the NM settings dict for openvpn connections:

connection.type = "vpn"
vpn.service-type = "org.freedesktop.NetworkManager.openvpn"
vpn.data = { connection-type, remote, port, proto-tcp, username, ca, cert, key, cipher, auth, tunnel-mtu, ... }
vpn.secrets = { password, cert-pass }
ipv4.method = "auto"   (server pushes config, unlike wireguard's "manual")
ipv6.method = "auto"

key difference from wireguard: openvpn uses the NM VPN plugin model with flat string dicts in vpn.data and vpn.secrets, not typed D-Bus properties.

tests

unit tests verifying correct keys/values in the output dict for each auth type.