Implement Block Header FinalSaplingRoot

[teor2345]

Is your feature request related to a problem? Please describe.

Describe the solution you'd like

• Implement the block::Commitment::FinalSaplingRoot variant based on the spec
  • it's the final Sapling root hash in the block, which we're already calculating, so we might as well verify it
  • blocked on Implement incremental, sparse note commitment trees for Sapling and Orchard #1287 - Sapling and Orchard root calculation

The Commitment enum variants have already been implemented.

Additional context

This historical validation rule was used in the Sapling and Blossom network upgrades. But it is replaced by the ChainHistoryRoot in Heartwood. Since Zebra checkpoints on Canopy, we do not need to implement this rule.

But it might be useful while we're testing ZIP-221 (#1567) or ZIP-244 (#2048).

[mpguerra]

Both ZIP-221 (#1567) or ZIP-244 (#2048) are already implemented. If we want to do this again at some point in the future we should re-open this issue.