Centralize anonymous user upgrade and handle upgrade conflicts #734

• Related to Inconsistent behavior of providers using GoogleProvider() and AppleProvider() #253
• Related to fix(ui_oauth_apple): upgrade anonymous users by linking the Apple credential #730

Problem

Anonymous users are upgraded inconsistently

When a user who signed in anonymously then signs in with a provider, FirebaseUI should link the provider to the anonymous account so their uid and data are kept. Whether that happens currently depends on the provider and platform:

• On mobile, most providers link the account; Apple did not until fix(ui_oauth_apple): upgrade anonymous users by linking the Apple credential #730.
• Email/password only links on sign-up. Email link only links when upgradeAnonymousUsers is enabled.
• On web, OAuth providers (Google, Apple, etc.) sign in as a new user instead of linking, so the anonymous user's data is lost. Phone sign-in on web links even when the signed in user is not anonymous.

Returning users can get stuck

If an anonymous user signs in with a provider account that already belongs to another user (for example, a returning user on a new device), linking fails with credential-already-in-use and the sign-in screen shows an error. The user cannot sign in with that provider while signed in anonymously, and the app has no way to recover the sign-in or merge the anonymous user's data.

Expected behaviour

• Upgrading anonymous users behaves the same for every provider and platform.
• When the account already exists, the app is given what it needs to sign the user in and merge the anonymous user's data, instead of a dead-end error.