[#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.go

@@ -0,0 +1,22 @@
+package subnetevents
+
+import (
+	"fmt"
+
+	subnetid "github.com/nspcc-dev/neofs-sdk-go/subnet/id"
+)
+
+// common interface of subnet notifications with subnet ID.
+type eventWithID interface {
+	// ReadID reads identifier of the subnet.
+	ReadID(*subnetid.ID) error
+}
+
+// an error which is returned on zero subnet operation attempt.
+type zeroSubnetOp struct {
+	op string
+}
+
+func (x zeroSubnetOp) Error() string {
+	return fmt.Sprintf("zero subnet %s", x.op)
+}