[chore][extension/googleclientauth] Use correct type for create func (#37568)

<!--Ex. Fixing a bug - Describe the bug and how this fixes the issue.
Ex. Adding a feature - Explain what this achieves.-->
#### Description

<!-- Issue number (e.g. #1234) or full URL to issue, if applicable. -->

Uses `extension.Extension` as the return type of `createExtension`
function


#### Link to tracking issue

Needed for open-telemetry/opentelemetry-collector/pull/12208

Author: mx-psi

extension/googleclientauthextension/factory.go

@@ -24,7 +24,7 @@ func NewFactory() extension.Factory {
 	)
 }
 
-func createExtension(ctx context.Context, set extension.Settings, cfg component.Config) (component.Component, error) {
+func createExtension(ctx context.Context, set extension.Settings, cfg component.Config) (extension.Extension, error) {
 	eCfg := cfg.(*Config)
 	return googleclientauthextension.CreateExtension(ctx, set, &eCfg.Config)
 }