[#355] subnet: Implement types for the new messages

Define `subnet.Info` structure of corresponding message from NeoFS API
protocol. Provide field getters and setters. Implement
`StableMarshal` / `StableSize` methods of binary encoding. Implement
`ToGRPCMessage` / `FromGRPCMessage` method for testing and
potential transport.

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

subnet/test/generate.go

@@ -0,0 +1,17 @@
+package subnettest
+
+import (
+	refstest "github.com/nspcc-dev/neofs-api-go/v2/refs/test"
+	"github.com/nspcc-dev/neofs-api-go/v2/subnet"
+)
+
+func GenerateSubnetInfo(empty bool) *subnet.Info {
+	m := new(subnet.Info)
+
+	if !empty {
+		m.SetID(refstest.GenerateSubnetID(false))
+		m.SetOwner(refstest.GenerateOwnerID(false))
+	}
+
+	return m
+}