container: add version getter

This field is a part of the container structure and this field can be
useful if for whatever reason we change the container structure. It's also
useful for the REST gateway that returns this data to users.

Signed-off-by: Roman Khimov <roman@nspcc.ru>

container/container.go

@@ -552,3 +552,10 @@ func (x Container) AssertID(id cid.ID) bool {
 
 	return id2.Equals(id)
 }
+
+// Version returns the NeoFS API version this container was created with.
+func (x Container) Version() version.Version {
+	var v version.Version
+	_ = v.ReadFromV2(*x.v2.GetVersion()) // No, this can't fail for x.
+	return v
+}

container/container_test.go

@@ -48,6 +48,9 @@ func TestContainer_Init(t *testing.T) {
 
 	val.Init()
 
+	var ver = val.Version()
+	require.Equal(t, version.Current(), ver)
+
 	var msg v2container.Container
 	val.WriteToV2(&msg)
 
@@ -60,7 +63,6 @@ func TestContainer_Init(t *testing.T) {
 	verV2 := msg.GetVersion()
 	require.NotNil(t, verV2)
 
-	var ver version.Version
 	require.NoError(t, ver.ReadFromV2(*verV2))
 
 	require.Equal(t, version.Current(), ver)