Align sync auth with other SDKs

The current auth API is hard to new users to understand and use. I suggest that Realm JS is aligned with the other Realm SDKs.

• The current methods (registerWithProvider, login) are deprecated and to be removed as soon as possible.
• A new classRealm.Sync.Credentials is added. It will interface supported 3rd party provides, e.g., JWT, Google and Facebook.
• Methods for logging in and and logging out are added to Realm.Sync.User.

An example:

credentials = Realm.Sync.Credentials.facebook(getFacebookToken())
Realm.Sync.User.login(credentials, "http://objectserver.realm.io/auth").then(user => {
    // user is now logged and and can open a Realm
}).catch(error => {
    // an error has occurred and be handled
})

[nirinchev]

This could be related: https://github.com/realm/realm-js/issues/1571

[kneth]

I close this as #2002 has been merged and released.