Add observatory / latestPing balancing strategy

Co-authored-by: Shelikhoo <xiaokangwang@outlook.com>

features/extension/contextreceiver.go

@@ -0,0 +1,7 @@
+package extension
+
+import "context"
+
+type ContextReceiver interface {
+	InjectContext(ctx context.Context)
+}

features/extension/observatory.go

@@ -0,0 +1,19 @@
+package extension
+
+import (
+	"context"
+
+	"github.com/golang/protobuf/proto"
+
+	"github.com/xtls/xray-core/features"
+)
+
+type Observatory interface {
+	features.Feature
+
+	GetObservation(ctx context.Context) (proto.Message, error)
+}
+
+func ObservatoryType() interface{} {
+	return (*Observatory)(nil)
+}