refactor(auth): Removed closure in login functions (#6235)
### Description
Removed the closure in login functions in favor of returning the token
and letting the caller determine exactly how to save it.
This should also help us with moving to `thiserror` since the error
typing story inside the closure was a little messy (should it take the
`turborepo_auth::Error` type or the `cli::Error` type?)
### Testing Instructions
Updated existing tests
Closes TURBO-1504
Co-authored-by: nicholaslyang <Nicholas Yang>
