[Front End] Async Validation create user check that email is not in use when loosing focus on email field

[brauliodiez]

Check that the email does not exist asynchronously in users, triggered when the email field loses focus

Step 1: The user enters an email in the "Create User" form
Step 2: The user loses focus on the email field
Step 3: Verify if the email exists in the database (we will use a Fonk validator and a specific endpoint for this)

Ask @manudous before starting this task