refactor: Change SignerData.SignerIndex to PubKey (#10692)

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## Description

Closes: #10691 

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- [ ] confirmed `!` in the type prefix if API or client breaking change
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client/tx/tx.go

@@ -247,24 +247,11 @@ func Sign(txf Factory, name string, txBuilder client.TxBuilder, overwriteSig boo
 		return err
 	}
 
-	pubkeys, err := txBuilder.GetTx().GetPubKeys()
-	if err != nil {
-		return err
-	}
-
-	signerIndex := 0
-	for i, p := range pubkeys {
-		if p.Equals(pubKey) {
-			signerIndex = i
-			break
-		}
-	}
-
 	signerData := authsigning.SignerData{
 		ChainID:       txf.chainID,
 		AccountNumber: txf.accountNumber,
 		Sequence:      txf.sequence,
-		SignerIndex:   signerIndex,
+		PubKey:        pubKey,
 		Address:       sdk.AccAddress(pubKey.Address()).String(),
 	}