[#11] accounting: Implement signing service

Adds accounting Service implementation that verifiers request signatures and
signs responses from internal Service.

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

cmd/neofs-node/config.go

@@ -2,7 +2,10 @@ package main
 
 import (
 	"context"
+	"crypto/ecdsa"
 	"sync"
+
+	crypto "github.com/nspcc-dev/neofs-crypto"
 )
 
 type cfg struct {
@@ -11,12 +14,18 @@ type cfg struct {
 	wg *sync.WaitGroup
 
 	grpcAddr string
+
+	key *ecdsa.PrivateKey
 }
 
 func defaultCfg() *cfg {
+	key, err := crypto.LoadPrivateKey("Kwk6k2eC3L3QuPvD8aiaNyoSXgQ2YL1bwS5CP1oKoA9waeAze97s")
+	fatalOnErr(err)
+
 	return &cfg{
 		ctx:      context.Background(),
 		wg:       new(sync.WaitGroup),
 		grpcAddr: "127.0.0.1:50501",
+		key:      key,
 	}
 }