fix: ensure signatures are ordered correctly (#12165)

web3.js/src/message.js

@@ -83,6 +83,16 @@ export class Message {
     );
   }
 
+  findSignerIndex(signer: PublicKey): number {
+    const index = this.accountKeys.findIndex(accountKey => {
+      return accountKey.equals(signer);
+    });
+    if (index < 0) {
+      throw new Error(`unknown signer: ${signer.toString()}`);
+    }
+    return index;
+  }
+
   serialize(): Buffer {
     const numKeys = this.accountKeys.length;