Function encode_extension 

pub fn encode_extension(
    oid: &[u64],
    payload_bytes: impl Into<Vec<u8>>,
) -> CustomExtension

Encodes an opaque DER payload into a non-critical custom X.509 extension.

The returned extension carries payload_bytes verbatim under oid. The payload is treated as opaque transport — no structure is imposed on or read from it here. Criticality is set to false so parsers that do not recognise oid will not reject the certificate.