The ID of the provider used to sign in the user.
One of "anonymous", "password", "facebook.com", "github.com",
"google.com", "twitter.com", "apple.com", "microsoft.com",
"yahoo.com", "phone", "playgames.google.com", "gc.apple.com",
or "custom".
The type identifier or factorId of the second factor, provided the
ID token was obtained from a multi-factor authenticated user.
For phone, this is "phone".