Add go_package on all proto files

Seems it is no longer optional after migratring off from
gogo/protobuf (see #6564).

Signed-off-by: Kazuyoshi Kato <katokazu@amazon.com>

api/next.pb.txt

@@ -48,6 +48,9 @@ file {
     type: TYPE_BOOL
     json_name: "fieldpath"
   }
+  options {
+    go_package: "github.com/containerd/containerd/protobuf/plugin"
+  }
 }
 file {
   name: "github.com/containerd/containerd/api/events/container.proto"
@@ -1214,6 +1217,9 @@ file {
       number: 5
     }
   }
+  options {
+    go_package: "github.com/containerd/containerd/api/types/task"
+  }
   syntax: "proto3"
 }
 file {