DO NOT MERGE: SBT signer sync

[SupremoUGH]

Adds the SBT signer synchronization method. The new sbt_sync updates the AssetMap and the Checkpoint, and returns a BalanceUpdate just like sync, but it doesn't update the UtxoAccumulator because you can't transfer SBTs so you don't need to generate membership proofs for them. This allows for much faster synchronization (with no Poseidon hashes computed) when dealing with SBTs.

Changes were included in: #336

DO NOT MERGE

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