Add possibility to set axios instance

[Hulkmaster]

What problem does this feature solve?

so in our project we have several api endpoints and we create several axios instances (with different baseurl and other settings) and auth should use one of them

What does the proposed changes look like?

// plugin-auth-extends.js

export default ({ $auth, $authApiHttpClient }) => {
  $auth.setHttpClient($authApiHttpClient);
}

_{This feature request is available on Nuxt community (#c573)}