TUN-4596: Add QUIC application protocol for QUIC stream handshake

- Vendored the capnproto library to cloudflared.
- Added capnproto schema defining application protocol.
- Added Pogs and application level read write of the protocol.

quic/pogs.go

@@ -0,0 +1,88 @@
+package quic
+
+import (
+	capnp "zombiezen.com/go/capnproto2"
+	"zombiezen.com/go/capnproto2/pogs"
+
+	"github.com/cloudflare/cloudflared/quic/schema"
+)
+
+// ConnectionType indicates the type of underlying connection proxied within the QUIC stream.
+type ConnectionType uint16
+
+const (
+	ConnectionTypeHTTP ConnectionType = iota
+	ConnectionTypeWebsocket
+	ConnectionTypeTCP
+)
+
+// ConnectRequest is the representation of metadata sent at the start of a QUIC application handshake.
+type ConnectRequest struct {
+	Dest     string         `capnp:"dest"`
+	Type     ConnectionType `capnp:"type"`
+	Metadata []Metadata     `capnp:"metadata"`
+}
+
+// Metadata is a representation of key value based data sent via RequestMeta.
+type Metadata struct {
+	Key string `capnp:"key"`
+	Val string `capnp:"val"`
+}
+
+func (r *ConnectRequest) fromPogs(msg *capnp.Message) error {
+	metadata, err := schema.ReadRootConnectRequest(msg)
+	if err != nil {
+		return err
+	}
+	return pogs.Extract(r, schema.ConnectRequest_TypeID, metadata.Struct)
+}
+
+func (r *ConnectRequest) toPogs() (*capnp.Message, error) {
+	msg, seg, err := capnp.NewMessage(capnp.SingleSegment(nil))
+	if err != nil {
+		return nil, err
+	}
+
+	root, err := schema.NewRootConnectRequest(seg)
+	if err != nil {
+		return nil, err
+	}
+
+	if err := pogs.Insert(schema.ConnectRequest_TypeID, root.Struct, r); err != nil {
+		return nil, err
+	}
+
+	return msg, nil
+}
+
+// ConnectResponse is a representation of metadata sent as a response to a QUIC application handshake.
+type ConnectResponse struct {
+	Error    string     `capnp:"error"`
+	Metadata []Metadata `capnp:"metadata"`
+}
+
+func (r *ConnectResponse) fromPogs(msg *capnp.Message) error {
+	metadata, err := schema.ReadRootConnectResponse(msg)
+	if err != nil {
+		return err
+	}
+	return pogs.Extract(r, schema.ConnectResponse_TypeID, metadata.Struct)
+}
+
+func (r *ConnectResponse) toPogs() (*capnp.Message, error) {
+	msg, seg, err := capnp.NewMessage(capnp.SingleSegment(nil))
+	if err != nil {
+		return nil, err
+	}
+
+	root, err := schema.NewRootConnectResponse(seg)
+	if err != nil {
+		return nil, err
+	}
+
+	if err := pogs.Insert(schema.ConnectResponse_TypeID, root.Struct, r); err != nil {
+		return nil, err
+	}
+
+	return msg, nil
+}