add getDER() to folly::AsyncTransportCertificate

Summary: Some implementations of this cert will need to access a DER representation of the Cert. So adding this to the interface.

Reviewed By: mingtaoy

Differential Revision: D64149926

fbshipit-source-id: f471ef71e042111aba67277cd3b0101e34f92169

quic/server/test/QuicServerTransportTest.cpp

@@ -5606,6 +5606,9 @@ class QuicServerTransportCertTest : public QuicServerTransportTest {
     std::string getIdentity() const override {
       return "";
     }
+    std::optional<std::string> getDER() const override {
+      return std::nullopt;
+    }
   };
 };