[#973] ir: Listen and process Put/Delete events of Subnet contract

Define notification events, implement parsers. Add morph client of
Subnet contract. Listen, verify and approve events in Inner Ring app.

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

pkg/innerring/processors/subnet/common_test.go

@@ -0,0 +1,19 @@
+package subnetevents
+
+import subnetid "github.com/nspcc-dev/neofs-sdk-go/subnet/id"
+
+type idEvent struct {
+	id subnetid.ID
+
+	idErr error
+}
+
+func (x idEvent) ReadID(id *subnetid.ID) error {
+	if x.idErr != nil {
+		return x.idErr
+	}
+
+	*id = x.id
+
+	return nil
+}