[WIP] Different extension's serialization names depending on the version

[olperr1]

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)

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?

What is the new behavior (if this is a feature change)?
Add several improvements on the extension serialization:

• It is now possible to use another serialization name and/or XML namespace prefix when exporting an extension.
• Previously, the XML namespace prefix could only have one value for an extension, determined by its SerDe class. Therefore, trying to import an XIIDM network containing an extension with a namespace prefix that didn't match the known prefix would fail. Now, the prefix could have any value supported by the XML format. (The prefix defined in the SerDe is now only used as a preferred one at export.)
• When exporting a network in XIIDM, if several extensions have the same namespace prefix in their SerDe (which could happen when using custom extensions), duplicates are renamed. For instance, if several extensions have the same prefix 'foo', the first one will be named 'foo' and the others 'foo1', 'foo2', ...

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:

[sonarqubecloud]

Quality Gate passed

Issues
4 New issues
0 Accepted issues

Measures
0 Security Hotspots
93.6% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud