GetUserByEmail and/or getUsers should return all users[FR]

**Is your feature request related to a problem? Yes**
It is very hard to search all the users by email when you allow the user to link multiple providers or if you allow multiple accounts with the same email.

**Describe the solution you'd like**
- The function "getUserByEmail" should return the first user that find where the email matches any of the emails found in its providers.
- The function "getUsers" should return a list of all the users that match that email in any of its providers.

**Describe alternatives you've considered**
We've been using a property to store the email as a record with the user data in firestore. But if you have any bug in any of your versions that list will break. It even becomes harder when you allow for multiple accounts and multiple provider linking in the same account.

We've consider using auth triggers in functions to keep that list updated, but there are no triggers when a user updates their auth credentials.

**Additional context**
Also as the feature "fetchSignInForEmail" and others like it are being removed for security purposes this feature is going to become even more useful and needed, and the admin is the perfect place to do it.

Also this is the behavior you get when searching for an email in the firebase auth panel.

Thank you very much for the great work. If I can help in any way I'll be happy to :)
Hope you have a great day.

Inizia individuando le implementazioni Admin Auth di getUserByEmail e getUsers, quindi esamina come vengono attualmente confrontate le email dei provider. Confronta il loro comportamento con quello del pannello Firebase Auth descritto nell'issue. Il lavoro è completato quando getUserByEmail restituisce il primo utente corrispondente e getUsers restituisce ogni utente la cui email corrisponde tra tutti i provider, con test che coprano più provider e account duplicati.

Scritto dal modello di indicizzazione a partire dal testo della issue.