[auth] Fix import/export path on useIdToken

[vwxyutarooo]

There are some missing/incorrect paths on auth, it won't work as the documentation expects.

• Fix import path of util from dist to source
• Add export path of auth/useIdToken.ts into auth/index.ts

[chrisbianca]

Thanks @vwxyutarooo, this was an oversight on my part. I've merged and will get a patch release out now