Store mapping of inbound tx - expected ballot for easier client-side CCTX tracking

[fadeev]

Currently, there is no API to get a ballot identifier directly from an inbound transaction hash. The only way to retrieve it is by reconstructing the inbound vote message (as zetatool does) using source-chain data.

Adding a storage mapping between inbound transactions and their expected ballot would make it possible to query ballot identifiers from ZetaChain directly, without querying connected chains' RPCs.

[lumtis]

Need to consider the implication for state growth as well since a new mapping would be added for inbounds but this might be minor compared to other modules.