Allow identity token creation to include optional identity attributes

[tldev]

Reference: https://docs.layer.com/sdk/web/authentication#identity-token

[tjohnell-handy]

@cakejelly - Any interest in getting this in any time soon? Going to fork the repo if not.

[cakejelly]

@tjohnell-handy i'm currently on vacation but i'll try to review the changes today at some point