Function decode_owned 

pub fn decode_owned(
    data: &mut Vec<u8>,
    ignore_garbage: bool,
    out: &mut impl Write,
) -> Result<()>

Decode base64 from an owned Vec (in-place whitespace strip + decode). Avoids a full buffer copy by stripping whitespace in the existing allocation, then decoding in-place. Ideal when the caller already has an owned Vec.