expose the QUIC version of a connection (#3620)

* Expose the QUIC version of a connection via connection state

* Update interface.go

Co-authored-by: Marten Seemann <martenseemann@gmail.com>

* Finish rename

* gofmt

* Use public type for version number

Co-authored-by: Marten Seemann <martenseemann@gmail.com>

interface.go

@@ -331,6 +331,7 @@ type Config struct {
 type ConnectionState struct {
 	TLS               handshake.ConnectionState
 	SupportsDatagrams bool
+	Version           VersionNumber
 }
 
 // A Listener for incoming QUIC connections