[#425] services: Define service interfaces that was removed from API lib

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

pkg/services/session/server.go

@@ -0,0 +1,12 @@
+package session
+
+import (
+	"context"
+
+	"github.com/nspcc-dev/neofs-api-go/v2/session"
+)
+
+// Server is an interface of the NeoFS API Session service server
+type Server interface {
+	Create(context.Context, *session.CreateRequest) (*session.CreateResponse, error)
+}