Function from_opt_str_or_number 

pub fn from_opt_str_or_number<'de, D, T>(
    deserializer: D,
) -> Result<Option<T>, D::Error>
where
    D: Deserializer<'de>,
    T: Deserialize<'de> + FromStr,
    <T as FromStr>::Err: Display,

Deserialize a value from an optional string or a number