FR: possibility to deactivate SignUp when signing in with email link

### [REQUIRED] Describe your environment

Firebase Auth, latest version.

### [REQUIRED] Describe the problem

`sendSignInLinkToEmail(email, settings)` does not only sign the user in but also registers him when his email does not exist yet.

The problem is that this behavior could undermine an extra registration process, in which the user has to specify more data than just the email.

#### Steps to reproduce:

Use sendSignInLinkToEmail(email, settings) with a user email that has not been registered yet.

#### FEATURE IDEA

The automatic sign up can be disabled in the firebase console. When a user tries to login with an email that has not been registered yet firebase returns an error which can be catched.

#### Relevant Code:

https://stackoverflow.com/questions/58652045/why-does-sendsigninlinktoemailemail-settings-not-check-whether-the-given-emai