The existing GoogleEmail2 auth did that:
- Acquire user access token with offline access
- Use token to acquire user's profile
- Build `Creds` with user's email as `ident` and the other profile
details as stringy key-value pairs in `credsExtra`
This wasn't enough for me, for several reasons:
- Access token was not saved after authentication. If we request 'offline'
token why not to have a way of using it later.
- Stringy key-value profile is not nice and `credsExtra` can be accessed
only from `getAuthId`
- I might want to request the profile after authentication process
So I've added the needed features.
- The access token is saved in a session
- There is a `Person` type with `FromJSON` instance and `getPerson`
can be used to acquire it from `HandlerT`
