Support private repository extensions

[igorbrites]

Feature description 💡

I created a private terrakube-extensions, and now I need to use it on the executors, though it's showing this error to me:

ERROR org.terrakube.executor.service.scripts.ScriptEngineService - https://github.com/MyOrg/terrakube-extensions: Authentication is required but no CredentialsProvider has been registered

How do I pass the credentials? I thought the same GitHub App used to make Dex work was also being used here, but it's not.

Anything else?

Opened as a feature request because idk if it's a bug 😬