Add onLogin & onLogout hooks

[fabhed]

What problem does this feature solve?

It allows for users to add custom functionality in one place.

What does the proposed changes look like?

export default function({ $auth }) {
  $auth.onLogin((strategy, data) => {
    // Do stuff
  })

  $auth.onLogout(() => {
    // Do stuff
  })
}

_{This feature request is available on Nuxt community (#c616)}

[NickBolles]

you can watch Logged in -> https://auth.nuxtjs.org/api/storage.html#local-state

I did find an issue with this callback not being Awaited though. So be warned about that. If this is implemented, implenting it as an awaitable stack would be awesome (or even something like KOAs middleware cascading)