[#311] control: Implement JSON marshaler on Netmap message

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

pkg/services/control/types.go

@@ -2,6 +2,7 @@ package control
 
 import (
 	"github.com/nspcc-dev/neofs-api-go/util/proto"
+	"google.golang.org/protobuf/encoding/protojson"
 )
 
 // SetKey sets public key used for signing.
@@ -309,3 +310,9 @@ func (x *Netmap) StableSize() int {
 
 	return size
 }
+
+func (x *Netmap) MarshalJSON() ([]byte, error) {
+	return protojson.MarshalOptions{
+		EmitUnpopulated: true,
+	}.Marshal(x)
+}