Function plan_extension 

pub fn plan_extension(
    underlying: &str,
    current_spot: f64,
    ref_price_at_listing: f64,
    expiry_timestamp: i64,
    existing_strikes: &[f64],
    strike_config: &StrikeSelectionConfig,
    extension_config: &ExtensionPolicyConfig,
) -> Result<ExtensionPlan>