add NetworkManager::import_ovpn() and OpenVpnBuilder::from_ovpn_file()

[cachebag]

part of #288. depends on the ovpn parser + cert store.

changes

add to OpenVpnBuilder:

pub fn from_ovpn_file(path: impl AsRef<Path>) -> Result<Self, ConnectionError>;
pub fn from_ovpn_str(content: &str, name: impl Into<String>) -> Result<Self, ConnectionError>;

parses the file, extracts inline certs (writing them via cert store), and pre-populates the builder. caller can override settings before .build().

add to NetworkManager:

pub async fn import_ovpn(
    &self,
    path: impl AsRef<Path>,
    username: Option<&str>,
    password: Option<&str>,
) -> Result<()>;

parses, creates the connection profile, and activates it.

tests

integration test: import a test .ovpn, verify NM connection profile has correct settings.