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

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## Description

Closes: #10691 

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---

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- [ ] included the correct [type prefix](https://github.com/commitizen/conventional-commit-types/blob/v3.0.0/index.json) in the PR title
- [ ] added `!` to the type prefix if API or client breaking change
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- [ ] added a changelog entry to `CHANGELOG.md`
- [ ] included comments for [documenting Go code](https://blog.golang.org/godoc)
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- [ ] confirmed the correct [type prefix](https://github.com/commitizen/conventional-commit-types/blob/v3.0.0/index.json) in the PR title
- [ ] confirmed `!` in the type prefix if API or client breaking change
- [ ] confirmed all author checklist items have been addressed 
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- [ ] reviewed tests and test coverage
- [ ] manually tested (if applicable)

x/auth/client/cli/tx_multisign.go

@@ -127,13 +127,13 @@ func makeMultiSignCmd() func(cmd *cobra.Command, args []string) (err error) {
 				return fmt.Errorf("set the chain id with either the --chain-id flag or config file")
 			}
 
-			for j, sig := range sigs {
+			for _, sig := range sigs {
 				signingData := signing.SignerData{
 					Address:       sdk.AccAddress(sig.PubKey.Address()).String(),
 					ChainID:       txFactory.ChainID(),
 					AccountNumber: txFactory.AccountNumber(),
 					Sequence:      txFactory.Sequence(),
-					SignerIndex:   j,
+					PubKey:        sig.PubKey,
 				}
 
 				err = signing.VerifySignature(sig.PubKey, signingData, sig.Data, txCfg.SignModeHandler(), txBuilder.GetTx())
@@ -328,7 +328,7 @@ func makeBatchMultisignCmd() func(cmd *cobra.Command, args []string) error {
 				ChainID:       txFactory.ChainID(),
 				AccountNumber: txFactory.AccountNumber(),
 				Sequence:      txFactory.Sequence(),
-				SignerIndex:   i,
+				PubKey:        pubKey,
 			}
 
 			for _, sig := range signatureBatch {

x/auth/client/cli/validate_sigs.go

@@ -110,7 +110,7 @@ func printAndValidateSigs(
 				ChainID:       chainID,
 				AccountNumber: accNum,
 				Sequence:      accSeq,
-				SignerIndex:   i,
+				PubKey:        pubKey,
 			}
 			err = authsigning.VerifySignature(pubKey, signingData, sig.Data, signModeHandler, sigTx)
 			if err != nil {