FR: Add method to either link a credential or sign in using it

### Feature proposal

* Firebase Component: Auth

Add a method which allows either signing in to an existing account, or linking a credential to the current account, in one call.

For example, a new method in FIRUser

```
- (void) linkOrSignInWithCredential:(FIRAuthCredential *)credential completion:(nullable void (^)(FIRAuthDataResult *_Nullable authResult, NSError *_Nullable error))completion {
//First try linking the given credential to the current user, and invoke the completion with the current user
//If that fails because the credential is already linked to another user, then log in as that user and invoke the completion with the new user
}
```

This cannot be completed currently because certain credentials, eg. OAuth credentials generated from signing in with Apple, may only be used once, due to the requirement of a nonce. The requested behavior is only possible in-app by asking the user to log in twice if the credential is already linked to another user.