Add finalize instruction to Record program

[karl-dz]

Add another instruction to finalize a record, which should prevent the original author from modifying a record.

The simple implementation would be to set the write authority to Pubkey::default().

Note of caution

For offchain processes like the validator revenue and reward calculators, we need to be extra careful about when these records are finalized. Finalizing a record should happen in conjunction with finalizing merkle roots on the Revenue Distribution.