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 — 87 passed, 2 skipped
   ⛔   Architecture Tests: ✅ successful — 17 passed, 2 skipped

Coverage for New Code

   〰️   Line Coverage: 100.00% — perfect 🎉
   ➰   Branch Coverage: 100.00% — perfect 🎉
     Mutation Coverage: 100.00% — perfect 🎉
   💪   Test Strength: 100.00% — perfect 🎉

Coverage for Whole Project

   〰️   Line Coverage: 91.57% — 65 missed lines
   ➰   Branch Coverage: 90.45% — 21 missed branches
     Mutation Coverage: 91.61% — 26 survived mutations
   💪   Test Strength: 94.67% — 16 survived mutations in tested code

Style

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

Bugs

     SpotBugs: No bugs
   🐛   Error Prone: No bugs

API Problems

   🚫   Revapi: No warnings

Vulnerabilities

   🛡️   OWASP Dependency Check: No vulnerabilities

Software Metrics

   🌀   Cyclomatic Complexity: 384 (total)
   💭   Cognitive Complexity: 174 (total)
   ➿   N-Path Complexity: 348 (total)
   📏   Lines of Code: 4412 (total)
   📝   Non Commenting Source Statements: 1274 (total)
   🔗   Class Cohesion: 71.43% (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
• ✅ Mutation 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.