CGMES extensions SerDe fixes

[flo-dup]

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, but should solve issue powsybl/pypowsybl#830 in other repository

What kind of change does this PR introduce?
Bug fix

What is the current behavior?

• serialization/deserialization unit tests missing on some CGMES extensions
• roundtrips in CGMES extensions / ENTSOE extension not using binary format
• Enum written as string (causes binary roundtrip to fail)
• attribute read in another order than written (causes json & binary roundtrip to fail)
• optional String attributes written using Optional::ifPresent instead of TreeDataWriter::writeOptionalString (causes binary roundtrip to fail)

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

• Yes
• No

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud