Connection.peer_cert_chain() is not exposed via FFI

As of version 0.21.0, [Connection.peer_cert](https://docs.rs/quiche/0.21.0/quiche/struct.Connection.html#method.peer_cert) is exposed via FFI as `quiche_conn_peer_cert` but [Connection.peer_cert_chain](https://docs.rs/quiche/0.21.0/quiche/struct.Connection.html#method.peer_cert_chain) isn't.

Could an extra `quiche_conn_peer` FFI function be added too?

Start by comparing the existing FFI exposure of Connection.peer_cert, documented as quiche_conn_peer_cert, with the Connection.peer_cert_chain API in the linked Rust documentation. Done means peer_cert_chain is exposed through a corresponding FFI function with behavior consistent with the existing peer-certificate binding.

Written by the indexing model from the issue text.