Write ACK_EXTENDED frame when supported by peer

Summary:
Write the new ACK_EXTENDED frame. When any of the ACK_EXTENDED features are enabled locally and supported by the peer, this frame type will take precedence over ACK_ECN and ACK_RECEIVETIMESTAMPS. See first diff in the stack for the features.

Frame format:
```
ACK_EXTENDED Frame {
  Type (i) = 0xB1
  // Fields of the existing ACK (type=0x02) frame:
  Largest Acknowledged (i),
  ACK Delay (i),
  ACK Range Count (i),
  First ACK Range (i),
  ACK Range (..) ...,
  Extended Ack Features (i),
  // Optional ECN counts (if bit 0 is set in Features)
  [ECN Counts (..)],
  // Optional Receive Timestamps (if bit 1 is set in Features)
  [Receive Timestamps (..)]
}
// Fields from the existing ACK_ECN frame
ECN Counts {
  ECT0 Count (i),
  ECT1 Count (i),
  ECN-CE Count (i),
}
// Fields from the existing ACK_RECEIVE_TIMESTAMPS frame
Receive Timestamps {
  Timestamp Range Count (i),
  Timestamp Ranges (..) ...,
}
Timestamp Range {
  Gap (i),
  Timestamp Delta Count (i),
  Timestamp Delta (i) ...,
}
```

Reviewed By: sharmafb

Differential Revision: D68931148

fbshipit-source-id: 0fb4bac23e121f82a11602daabc1ec7084db43dd

Author: jbeshay

proxygen/httpserver/samples/hq/HQCommandLine.cpp

@@ -121,14 +121,20 @@ DEFINE_uint32(
     max_ack_receive_timestamps_to_send,
     quic::kMaxReceivedPktsTimestampsStored,
     "Controls how many packet receieve timestamps the peer should send");
-DEFINE_uint32(
-    extended_ack_features,
-    0,
-    "Replace the ACK frame with ACK_EXTENDED with the following features."
-    "The bitwise values can be ORed together."
-    "bit 1 - ECN support"
-    "bit 2 - Receive timestamps support"
-    "Example: 3 means both ECN and receive timestamps are supported");
+DEFINE_uint32(advertise_extended_ack_features,
+              0,
+              "Advertise ACK_EXTENDED frame support to the peer. The following"
+              "bitwise values can be ORed together:"
+              "bit 1 - ECN support"
+              "bit 2 - Receive timestamps support"
+              "Example: 3 means both ECN and receive timestamps are supported");
+DEFINE_uint32(enable_extended_ack_features,
+              0,
+              "Replace the ACK frame with ACK_EXTENDED when supported by the "
+              "peer. The following bitwise values can be ORed together:"
+              "bit 1 - ECN support"
+              "bit 2 - Receive timestamps support"
+              "Example: 3 means both ECN and receive timestamps are supported");
 DEFINE_bool(initiate_key_updates,
             false,
             "Whether to initiate periodic key updates");
@@ -316,7 +322,9 @@ void initializeTransportSettings(HQToolParams& hqUberParams) {
   hqParams.transportSettings.disableMigration = false;
 
   hqParams.transportSettings.advertisedExtendedAckFeatures =
-      FLAGS_extended_ack_features;
+      FLAGS_advertise_extended_ack_features;
+  hqParams.transportSettings.enableExtendedAckFeatures =
+      FLAGS_enable_extended_ack_features;
 } // initializeTransportSettings
 
 void initializeHttpServerSettings(HQToolServerParams& hqParams) {