Support for multiple accounts

**Is your feature request related to a problem you're having? Please describe.**
Yes. I have an application that supports multiple gmail accounts. I would like Google-SignIn to retain the login state of multiple accounts at once that way my users can switch back and forth between their gmail accounts without being prompted to login each time.

For example:
User A has 2 gmail addresses. My application allows user A to login to both accounts and switch back and forth between those accounts at a click of a button.

**Describe the solution you'd like**
currentUser could be an array of GIDGoogleUser. Each user points to a different Google account. There could be a function `currentUser(with id:)` that would let me get a specific user.
`restorePreviousSignIn` would return an array of GIDGoogleUser instead of a single user.
Each time I would call `GIDSignIn.sharedInstance.signIn`, it would add a new user to currentUser.
And so on.

**Describe alternatives you've considered**
I thought about using an OAuth library instead.

**Additional context**
None.