Please check if the PR fulfills these requirements

• The commit message follows our guidelines
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)
• A PR or issue has been opened in all impacted repositories (if any)

Does this PR already have an issue describing the problem?

No

What kind of change does this PR introduce?

Feature

What is the current behavior?

In the voltage regulation refactoring process, some data that were previously in extensions are moved into the IIDM model. So the corresponding extensions are not existing anymore.
But for backward-compatibility, these extensions must be present in the network files (XIIDM, JIIDM and BIIDM) exported to the previous IIDM versions.

In the first commits of #3699, the serialization was handled "manually" in NetworkSerDe.

What is the new behavior (if this is a feature change)?
This PR introduces a generic mechanism to handle the backward-compatibility of the serialization of the "extinct" extensions.

Does this PR introduce a breaking change or deprecate an API?

• Yes
• No

If yes, please check if the following requirements are fulfilled

• The Breaking Change or Deprecated label has been added
• The migration steps are described in the following section

What changes might users need to make in their application due to this PR? (migration steps)

Other information: