Make the TTree based ROOT backend store the GenericParameters the same way as the RNTuple based one

[tmadlener]

Superseded by #625

• Fixes #590
• [ ] Builds on #614
• [x] Includes #623
• [x] Needs #580 in order to make the edm4hep tests pass again
• [x] Add an additional "legacy test file" for v0.99