Make quic knobs available in mobileconfigs

Summary:
After discussion with mjoras, it seems a good idea to make a special knob
space for quic's internal uses. This is the first step towards that. I use json
as the serialized format because it's fairly easy to write/parse, and probably more
reliable than implementing our own special syntax. ~~Currently we should use
numbers only, but we can make it easier for config writers by reserving a
predefined map<string, int> for commonly used knob spaces and knob ids.~~

~~For example, for d6d's blackhole detection threshold we could have it as
`{"quic_transport": { "d6d_blackhole_detection_threshold":  5 }}`. And we can store
the mapping somewhere convenient for mobile clients (perhaps another string  in
mobile config?).~~
```
"quic_knob_spaces": {
  "quic_transport" : {
     "space_id" : 0,
     "knob_ids" : {
        "d6d_blackhole_detection_threshold" : 0,
        ...
     }
  }
}
```

Reviewed By: mjoras

Differential Revision: D23822004

fbshipit-source-id: ddb60b86c3a8eb54ae0a011a2e041e096e14cb9e

quic/QuicConstants.h

@@ -103,6 +103,12 @@ constexpr uint64_t kDefaultBufferSpaceAvailable =
 constexpr std::chrono::microseconds kDefaultMinRtt =
     std::chrono::microseconds::max();
 
+// Default knob space for transport knobs
+constexpr uint64_t kDefaultQuicTransportKnobSpace = 0xfaceb001;
+
+// Default knob id for transport knobs
+constexpr uint64_t kDefaultQuicTransportKnobId = 1;
+
 enum class FrameType : uint64_t {
   PADDING = 0x00,
   PING = 0x01,

quic/state/TransportSettings.h

@@ -108,6 +108,13 @@ struct D6DConfig {
   ProbeSizeRaiserType raiserType{ProbeSizeRaiserType::ConstantStep};
 };
 
+// JSON-serialized transoprt knobs
+struct SerializedKnob {
+  uint64_t space;
+  uint64_t id;
+  folly::StringPiece blob;
+};
+
 struct TransportSettings {
   // The initial connection window advertised to the peer.
   uint64_t advertisedInitialConnectionWindowSize{kDefaultConnectionWindowSize};
@@ -237,6 +244,8 @@ struct TransportSettings {
   bool orderedReadCallbacks{false};
   // Config struct for D6D
   D6DConfig d6dConfig;
+  // Quic knobs
+  std::vector<SerializedKnob> knobs;
 };
 
 } // namespace quic