[#139] object: Implement function to init object creation

All NeoFS object must have at least container and owner identifiers.

Add `InitCreation` function which write all required fields to the
object instance. Extend `Object` type docs with all the constructors.

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

object/object_test.go

@@ -0,0 +1,24 @@
+package object_test
+
+import (
+	"testing"
+
+	cidtest "github.com/nspcc-dev/neofs-sdk-go/container/id/test"
+	"github.com/nspcc-dev/neofs-sdk-go/object"
+	ownertest "github.com/nspcc-dev/neofs-sdk-go/owner/test"
+	"github.com/stretchr/testify/require"
+)
+
+func TestInitCreation(t *testing.T) {
+	var o object.Object
+	cnr := *cidtest.ID()
+	own := *ownertest.ID()
+
+	object.InitCreation(&o, object.RequiredFields{
+		Container: cnr,
+		Owner:     own,
+	})
+
+	require.Equal(t, &cnr, o.ContainerID())
+	require.Equal(t, &own, o.OwnerID())
+}