Implement block witness lazy loading

https://github.com/0xMiden/protocol/pull/3699 added the `TransactionEventId::TxBeforeBlockWitnessLoad` so the tx exec host can check whether a block witness is present in the advice provider and if not, ask the `DataStore` to provide it. The event was added, but the handler is a no-op. The task is to implement the handler which consists of the witness presence check and the data store request, analogously to the other lazy loading events.

When we implement this, we should remove `MockTransactionBuilder::required_blocks` and the `required_blocks` parameter on `get_transaction_inputs_at` since this is only needed while we don't have lazy-loading and allows us to clean up the API.