Add sandbox events protos

Signed-off-by: Maksym Pavlenko <pavlenko.maksym@gmail.com>

api/events/sandbox.proto

@@ -0,0 +1,37 @@
+/*
+	Copyright The containerd Authors.
+
+	Licensed under the Apache License, Version 2.0 (the "License");
+	you may not use this file except in compliance with the License.
+	You may obtain a copy of the License at
+
+		http://www.apache.org/licenses/LICENSE-2.0
+
+	Unless required by applicable law or agreed to in writing, software
+	distributed under the License is distributed on an "AS IS" BASIS,
+	WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+	See the License for the specific language governing permissions and
+	limitations under the License.
+*/
+
+syntax = "proto3";
+
+package containerd.events;
+
+import "google/protobuf/timestamp.proto";
+
+option go_package = "github.com/containerd/containerd/api/events;events";
+
+message SandboxCreate {
+	string sandbox_id = 1;
+}
+
+message SandboxStart {
+	string sandbox_id = 1;
+}
+
+message SandboxExit {
+	string sandbox_id = 1;
+	uint32 exit_status = 2;
+	google.protobuf.Timestamp exited_at = 3;
+}