feat(store): cache account witnesses for tracked accounts

[sofiazcoaga]

Problem

Every FPI transaction fetches the foreign account's AccountWitness from the node. N transactions between two syncs cost N GetAccount requests, even against the same account at the same block.

Changes

1. Client::track_account_witness registers an account whose account witness should be kept fresh. New account_witnesses table, one row per account.
2. Every sync refreshes them at the new sync height, up to four requests in flight. Each witness is validated against the synced block header's account root before it is stored.
3. The sync reuses the witnesses it already validated while fetching public account state, so a registered account that changed in that window costs one request, not two.
4. Building a transaction, a registered foreign account resolves from the store instead of the network: a private one takes the cached witness, a public one the client tracks builds its whole AccountInputs locally. Both fall back to the existing request when there is nothing usable.

The cost moves from once per transaction to once per sync.

Closes #2443

[sofiazcoaga]

@igamigo the issue mentions "we should also keep account witnesses for watched accounts, which we already request anyway", but I deliberately did not add watched accounts to account witness fetching automatically. Watched accounts are only refetched during sync when they have been modified, so for an unmodified one there is no request to reuse. Automatically tracking their account witnesses would mean an RPC call for every watched account on every sync, so a client watching N accounts for observability would pay N extra requests per sync. I believe this should be a conscious decision. Let me know your thoughts on this.