Enable requirement of ALPN negotiation in H3 datagram connector

Summary:
H3DatagramAsyncSocket talks to fwdproxy through HTTP3 CONNECT-UDP tunnel. Both sides need to agree on a specific version of HTTP3 protocol. [Here](https://www.internalfb.com/code/fbsource/[7b9cbf3292c5]/fbcode/proxygen/lib/transport/H3DatagramAsyncSocket.cpp?lines=316-317) is the ALPN negotiation for that purpose. Since version mismatch will cause undefined behavior, I am enabling the requirement of ALPN negotiation.

By the way, I am also removing the code to set default shares per knekritz's suggestion. The client doesn't need to put multiple key shares in the client hello. The default ```x25519``` works as expected.

Differential Revision: D62574965

fbshipit-source-id: 5bc8f5fd27ddd9b10f6039bec1f5653ddd9d09ea

proxygen/lib/transport/H3DatagramAsyncSocket.cpp

@@ -315,8 +315,7 @@ H3DatagramAsyncSocket::createFizzClientContext() {
 
   std::vector<std::string> supportedAlpns = {proxygen::kH3FBCurrentDraft};
   ctx->setSupportedAlpns(supportedAlpns);
-  ctx->setDefaultShares(
-      {fizz::NamedGroup::x25519, fizz::NamedGroup::secp256r1});
+  ctx->setRequireAlpn(true);
   ctx->setSendEarlyData(false);
   return ctx;
 }