Restore serialization to use interfaces again

[uhafner]

Changing the fields to serializable classes breaks the existing serializations.

[github-actions]

☀️   Quality Monitor

Tests

     Unit Tests: ✅ successful — 190 passed
   🚀   Integration Tests: ✅ successful — 148 passed, 7 skipped
   ⛔   Architecture Tests: ✅ successful — 15 passed

Coverage for New Code

   〰️   Line Coverage: 100.00% — perfect 🎉
   ➰   Branch Coverage: 100.00% — perfect 🎉

Coverage for Whole Project

   〰️   Line Coverage: 76.24% — 727 missed lines
   ➰   Branch Coverage: 66.63% — 292 missed branches

Style

     CheckStyle: No warnings
     PMD: No warnings
   ☕   Java Compiler: No warnings

Bugs

     SpotBugs: No bugs
   🐛   Error Prone: No bugs

Vulnerabilities

   🛡️   OWASP Dependency Check: 206 vulnerabilities — error: 18, high: 53, normal: 129, low: 6

Software Metrics

   🌀   Cyclomatic Complexity: 1028 (total)
   💭   Cognitive Complexity: 562 (total)
   ➿   N-Path Complexity: 1385 (total)
   📏   Lines of Code: 12044 (total)
   📝   Non Commenting Source Statements: 4058 (total)
   🔗   Class Cohesion: 100.00% (maximum)
   ⚖️   Weight of Class: 100.00% (maximum)

🚦 Quality Gates

Overall Status: ✅ SUCCESS

✅ Passed Gates

• ✅ Overall Tests Success Rate: 100.00 >= 100.00
• ✅ Line Coverage in New Code: 100.00 >= 90.00
• ✅ Branch Coverage in New Code: 100.00 >= 90.00
• ✅ Potential Bugs in Whole Project: 0.00 <= 0.00
• ✅ Style Violation in Whole Project: 0.00 <= 0.00

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Created by Quality Monitor v4.10.0 (#be1b382). More details are shown in the GitHub Checks Result.