Login user with actual attributes

[dburdan]

This PR updates loginUserWithUserAttributes to use IntercomAttributesBuilder when passing in additional attributes for a new user, just as updateUser does. This is iOS only as I haven't updated the Android method yet.

The parameter typing on loginUserWithUserAttributes shows the full UserAttributes object, but the underlying methods only retain userId and email and discard the rest, which is confusing when passing in additional data and not seeing it on the dashboard. Previously this also required two round-trips, one for initial login and another to update the created user's attributes.

If this isn't the desired behavior I'd be happy to instead update the loginUserWithUserAttributes types to require only userId or email.

[Br1an-Boyle]

Thanks for the PR 👍🏻