Add replacementBlockhash to the simulation return type, in preparat…

…ion for Agave 2.0 (#2870)

# Summary

When you ask the simulator to `replaceRecentBlockhash`, you may like to know what the replacement blockhash was. Agave 2.0 will return it in the simulation result.

It would have been nice to _only_ include this property when `replaceRecentBlockhash` is explicitly `true`, but I would have had to duplicate all of the overloads for the purpose, yielding 32 in total.

packages/rpc-api/src/simulateTransaction.ts

@@ -7,6 +7,7 @@ import type {
     AccountInfoWithJsonData,
     Base58EncodedBytes,
     Base64EncodedDataResponse,
+    Blockhash,
     Commitment,
     Slot,
     SolanaRpcResponse,
@@ -91,6 +92,11 @@ type SimulateTransactionApiResponseBase = SolanaRpcResponse<{
     err: TransactionError | null;
     /** Array of log messages the transaction instructions output during execution, null if simulation failed before the transaction was able to execute (for example due to an invalid blockhash or signature verification failure) */
     logs: string[] | null;
+    // TODO(https://github.com/solana-labs/solana-web3.js/issues/2869): Make it so that
+    // `replacementBlockhash` only appears on the return type when `replaceRecentBlockhash` is set
+    // to `true` in the call to `simulateTransaction`.
+    /** The blockhash that was used to simulate the transaction when `replaceRecentBlockhash` is `true` */
+    replacementBlockhash?: Blockhash;
     /** The most-recent return data generated by an instruction in the transaction */
     returnData: Readonly<{
         /** The return data itself, as base-64 encoded binary data */