[rntuple] Footer Envelope: Handle Linked Attribute Set Record Frames

[sathabbott]

A new record frame has been added to the Footer Envelope. Need to update rootfilespec to handle this new set of metadata.

https://github.com/root-project/root/blob/master/tree/ntuple/doc/BinaryFormatSpecification.md#linked-attribute-set-record-frame

[sathabbott]

reference these slides:
https://indico.cern.ch/event/1488410/contributions/6562809/attachments/3130274/5553188/RNTupleAttributes_acat2025.pdf