v0.39.3

- zkgroup: Add GroupSendCredentialResponse::receive_with_ciphertexts,
  which is exposed as an overload of receive() for Java and Swift, and
  as receiveWithCiphertexts() for TypeScript. This is more efficient
  if the client already has ciphertexts for the group members.

- Java: Expose ServerCertificate and SenderCertificate creation APIs,
  and add them to libsignal-server as well as libsignal-client.

- Update Cargo dependencies again.