Function extract_optional_string_field 

fn extract_optional_string_field<'a>(
    object: &'a CanonicalJsonObject,
    field: &'a str,
) -> Result<Option<&'a String>>

Extract the optional string field with the given name from the given canonical JSON object.

Returns Ok(Some(value)) if the field is present and a valid string, Ok(None) if the field is missing and Err(_) if the field is not a string or its length is bigger than [ID_MAX_BYTES].