[#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/morph/client/subnet/client.go

@@ -0,0 +1,91 @@
+package morphsubnet
+
+import (
+	"fmt"
+
+	"github.com/nspcc-dev/neo-go/pkg/util"
+	"github.com/nspcc-dev/neofs-node/pkg/morph/client"
+)
+
+// Client represents Subnet contract client.
+//
+// Client should be preliminary initialized (see Init method).
+type Client struct {
+	client *client.StaticClient
+}
+
+// InitPrm groups parameters of Client's initialization.
+type InitPrm struct {
+	base *client.Client
+
+	addr util.Uint160
+
+	modeSet bool
+	mode    Mode
+}
+
+// SetBaseClient sets basic morph client.
+func (x *InitPrm) SetBaseClient(base *client.Client) {
+	x.base = base
+}
+
+// SetContractAddress sets address of Subnet contract in NeoFS sidechain.
+func (x *InitPrm) SetContractAddress(addr util.Uint160) {
+	x.addr = addr
+}
+
+// Mode regulates client work mode.
+type Mode uint8
+
+const (
+	_ Mode = iota
+
+	// NonNotary makes client to work in non-notary environment.
+	NonNotary
+
+	// NotaryAlphabet makes client to use its internal key for signing the notary requests.
+	NotaryAlphabet
+
+	// NotaryNonAlphabet makes client to not use its internal key for signing the notary requests.
+	NotaryNonAlphabet
+
+	lastMode
+)
+
+// SetMode makes client to work with non-notary sidechain.
+// By default, NonNotary is used.
+func (x *InitPrm) SetMode(mode Mode) {
+	x.modeSet = true
+	x.mode = mode
+}
+
+// Init initializes client with specified parameters.
+//
+// Base client must be set.
+func (x *Client) Init(prm InitPrm) error {
+	if prm.base == nil {
+		panic("missing base morph client")
+	}
+
+	if !prm.modeSet {
+		prm.mode = NonNotary
+	}
+
+	var opts []client.StaticClientOption
+
+	switch prm.mode {
+	default:
+		panic(fmt.Sprintf("invalid work mode %d", prm.mode))
+	case NonNotary:
+	case NotaryNonAlphabet:
+		opts = []client.StaticClientOption{client.TryNotary()}
+	case NotaryAlphabet:
+		opts = []client.StaticClientOption{client.TryNotary(), client.AsAlphabet()}
+	}
+
+	var err error
+
+	x.client, err = client.NewStatic(prm.base, prm.addr, 0, opts...)
+
+	return err
+}