[#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/delete_test.go

@@ -0,0 +1,44 @@
+package subnetevents
+
+import (
+	"errors"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+
+	subnetid "github.com/nspcc-dev/neofs-sdk-go/subnet/id"
+)
+
+type delete struct {
+	idEvent
+}
+
+func TestDeleteValidator_Assert(t *testing.T) {
+	var (
+		v DeleteValidator
+
+		e delete
+
+		err error
+	)
+
+	// read ID error
+	e.idErr = errors.New("id err")
+
+	err = v.Assert(e)
+	require.ErrorIs(t, err, e.idErr)
+
+	e.idErr = nil
+
+	// zero subnet ID
+	subnetid.MakeZero(&e.id)
+
+	err = v.Assert(e)
+	require.ErrorAs(t, err, new(zeroSubnetOp))
+
+	const idNum = 13
+	e.id.SetNumber(idNum)
+
+	err = v.Assert(e)
+	require.NoError(t, err)
+}