[#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.go

@@ -0,0 +1,44 @@
+package subnetevents
+
+import (
+	"fmt"
+
+	subnetid "github.com/nspcc-dev/neofs-sdk-go/subnet/id"
+)
+
+// Delete represents notification about NeoFS subnet removal.
+// Generated by a contract when intending to delete a subnet.
+type Delete interface {
+	// Contains ID of the subnet to be removed.
+	eventWithID
+}
+
+// DeleteValidator asserts intent to remove a subnet.
+type DeleteValidator struct{}
+
+// Assert processes the attempt to remove a subnet. Approves the removal through nil return.
+//
+// All read errors of Delete are forwarded.
+//
+// Returns an error on:
+//   * zero subnet creation;
+//   * empty ID or different from the one wired into info;
+//   * empty owner ID or different from the one wired into info.
+func (x DeleteValidator) Assert(event Delete) error {
+	var err error
+
+	// read ID
+	var id subnetid.ID
+	if err = event.ReadID(&id); err != nil {
+		return fmt.Errorf("read ID: %w", err)
+	}
+
+	// prevent zero subnet removal
+	if subnetid.IsZero(id) {
+		return zeroSubnetOp{
+			op: "removal",
+		}
+	}
+
+	return nil
+}