support passing additional signers to sendTransaction

ts/client/src/client.ts

@@ -803,6 +803,7 @@ export class MangoClient {
       this.program.provider as AnchorProvider,
       transaction,
       {
+        additionalSigners,
         postSendTxCallback: this.postSendTxCallback,
       },
     );

ts/client/src/utils/rpc.ts

@@ -12,6 +12,9 @@ export async function sendTransaction(
     await connection.getLatestBlockhash(opts.preflightCommitment)
   ).blockhash;
   transaction.feePayer = payer.publicKey;
+  if (opts.additionalSigners.length > 0) {
+    transaction.partialSign(...opts.additionalSigners);
+  }
 
   await payer.signTransaction(transaction);
   const rawTransaction = transaction.serialize();