Function extend_valid_utf8 

fn extend_valid_utf8<'a, B, I>(builder: &mut B, iter: I)
where
    B: Extend<Option<&'a str>>,
    I: Iterator<Item = Option<&'a [u8]>>,

A specified helper to cast from GenericBinaryArray to GenericStringArray when they have same offset size so re-encoding offset is unnecessary.