OverloadManagementSystem extensions can't be deserialized when using the default impl.

[bc-rte]

Describe the current behavior

When trying to deserialize an IIDM file with an extension on OverloadManagementSystem, the OverloadManagementSystemSerDe implementation prevents the extension from being deserialized.

This is caused by OverloadManagementSystems' creation in the Network being delayed after the extension creation step.

Describe the expected behavior

the iidm implementation should let extensions on OverloadManagementSystem be deserialized.

Describe the steps

No response

Environment

No response

Relevant Log Output

No response

Extra Information

This is caused by OverloadManagementSystemSerDe's postponeElementCreation handling mechanism using toApply to create OverloadManagementSystem. toApply's implementation occurs at the end of the deserialization process, after extensions.

Ideally, this mechanism should only postpone validation of the element.

[bc-rte]

fix PR: #3171