openvpn tls hardening options

[cachebag]

part of #288.

changes

add fields to OpenVpnConfig and corresponding builder methods:

• tls_auth_key: Option<String> + tls_auth_direction: Option<u8>
• tls_crypt: Option<String>
• tls_crypt_v2: Option<String>
• tls_version_min: Option<String> / tls_version_max: Option<String>
• tls_cipher: Option<String>
• remote_cert_tls: Option<String> ("server" or "client")
• verify_x509_name: Option<(String, String)> (name, type)
• crl_verify: Option<String>

wire into build_openvpn_connection() — map each to the correct NM vpn.data key.

tests

unit tests for settings serialization of each tls option.