feat(aztec-nr)!: add explicit custom_sync_state hook to AztecConfig

[nchamo]

Why we are doing this

Contracts that wanted to customize state sync had to override the macro-generated sync_state utility by defining a function with that name. The override happened silently — the macro had no way to surface that the default was being replaced, and there was no compile-time signal when the override accidentally shadowed the generated function.

Our fix

Adds an explicit AztecConfig::custom_sync_state() builder that registers a CustomSyncHandler. The handler receives the same parameters as the default do_sync_state and can wrap, delegate to, or skip it. Defining a function named sync_state directly now errors with a message pointing users at the new builder.

The naming follows the existing CustomMessageHandler pattern for sibling consistency. A custom_sync_state_contract exercises the full flow (deploy + execute_utility) and verifies the handler is invoked in the right scope.

Migration

See migration_notes.md. Only contracts that previously defined their own sync_state function are affected; users of the default macro-generated sync need no changes.

Fixes F-655

[nchamo]

I wasn't sure if we should call this custom_sync_state() or custom_sync_handler(). What do you think?