Function build_preferred_sig_algs_extension 

pub(crate) fn build_preferred_sig_algs_extension(prefer_str: &str) -> Vec<u8> 

Build the DER-encoded PreferredSignatureAlgorithms extension value.

Uses the same #[derive(der::Sequence)] struct used for parsing in hoike-core, ensuring encode/decode symmetry. RFC 6960 §4.4.7.1.