Add v2 version to go module name

Replace all elements from `v2` to root directory.

Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>

container/json.go

@@ -0,0 +1,22 @@
+package container
+
+import (
+	container "github.com/nspcc-dev/neofs-api-go/v2/container/grpc"
+	"github.com/nspcc-dev/neofs-api-go/v2/rpc/message"
+)
+
+func (a *Attribute) MarshalJSON() ([]byte, error) {
+	return message.MarshalJSON(a)
+}
+
+func (a *Attribute) UnmarshalJSON(data []byte) error {
+	return message.UnmarshalJSON(a, data, new(container.Container_Attribute))
+}
+
+func (c *Container) MarshalJSON() ([]byte, error) {
+	return message.MarshalJSON(c)
+}
+
+func (c *Container) UnmarshalJSON(data []byte) error {
+	return message.UnmarshalJSON(c, data, new(container.Container))
+}