feat: SSH as proxy outbound

[XOR-op]

This PR enables SSH as a feasible proxy outbound. The configuration format is as below:

<PROXY-NAME>:
        server: String, #<SERVER-DOMAIN-OR-IP>
        port: u16,
        user: String,
        password: Option<String>,
        private-key: Option<PathBuf>,
        key-passphrase: Option<String>,
        host-pubkey: Option<SingleOrVec<String>>,

Some rules worth noticing:

• When private-key is present, password field will be ignored.
• When host-pubkey is empty, the pubkey check will be skipped without warning.