cmd/geth: rename variable 'extapi' (ethereum#27223)

rename parameter

In this case, the naming of "extapi" might create some confusion. Although it represents an External Signer Backend, its name could be mistaken for an API. In reality, it is a backend instance used for communicating with external signers. A better naming choice could be "extBackend" or "externalBackend" to more accurately describe that it is a backend instance rather than an API.

cmd/geth/config.go

@@ -288,8 +288,8 @@ func setAccountManagerBackends(conf *node.Config, am *accounts.Manager, keydir s
 	// Assemble the supported backends
 	if len(conf.ExternalSigner) > 0 {
 		log.Info("Using external signer", "url", conf.ExternalSigner)
-		if extapi, err := external.NewExternalBackend(conf.ExternalSigner); err == nil {
-			am.AddBackend(extapi)
+		if extBackend, err := external.NewExternalBackend(conf.ExternalSigner); err == nil {
+			am.AddBackend(extBackend)
 			return nil
 		} else {
 			return fmt.Errorf("error connecting to external signer: %v", err)