Mostly remove folly::Optional

Summary:
This is an API break, but it should mostly be a manageable one. We want to be able to compile mvfst internally without exceptions, and folly::Optional is one dependency that makes this challenging. Additionally, we already have an imported secondary optional type for performance/struct size reasons, tiny-optional.

This second optional interface is mostly compatible in an API sense (including the use of std::nullopt) with std::optional. Thus our approach is to remove the dependency on folly::Optional, and offer a quic::Optional instead.

The next diff will properly vendor tiny-optional so that quic::Optional is an independent version of it.

Reviewed By: sharmafb, kvtsoy

Differential Revision: D74133131

fbshipit-source-id: 715f8bb5043ba3bb876cacfe54236887e0686b30

quic/api/test/QuicStreamAsyncTransportTest.cpp

@@ -181,7 +181,7 @@ class QuicStreamAsyncTransportTest : public Test {
 
   void TearDown() override {
     if (client_) {
-      client_->close(none);
+      client_->close(std::nullopt);
     }
     clientEvb_.loop();
     server_->shutdown();