There should be an idempotent equivalent of createUser

[icopp]

For example, this could be a method like auth.admin.ensureUser that

• takes the same arguments as createUser
• returns a matching user if there is a single matching user
• updates the matching user to include additional params if there's a single matching user (e.g. add email or phone number if they don't correspond to any other users)
• returns an error if there's more than one matching user (such as email and phone corresponding to separate users)

This would make life much easier for those of us who have to deal with programmatically created users, since it would allow replacing current slower and more complicated code with a single call.