openvpn compression and proxy support

[cachebag]

part of #288.

compression

#[non_exhaustive]
pub enum OpenVpnCompression {
    No,
    Lzo,
    Lz4,
    Lz4V2,
    Yes,
}

maps to NM keys compress and/or comp-lzo.

proxy

#[non_exhaustive]
pub enum OpenVpnProxy {
    Http { server: String, port: u16, username: Option<String>, password: Option<String>, retry: bool },
    Socks { server: String, port: u16, retry: bool },
}

maps to NM keys: proxy-type, proxy-server, proxy-port, proxy-retry, http-proxy-username, http-proxy-password.

add builder methods and wire into build_openvpn_connection().

tests

unit tests for each compression mode and proxy type serialization.