[FR]: Add Email Link Authentication

[creeperkatze]

Description

1. Email Link Authentication would allow users to sign up or sign in easily without using a password, making it really simple and secure
2. While it is a feature of Firebase Authentication (Docs) the Unity SDK doesn't support it yet
3. Functions like SendSignInLinkToEmailAsync(), the deep linking could be handled by unity itself

API Proposal

API	Signature	Purpose
ActionCodeSettings	Class with properties: Url, HandleCodeInApp, AndroidPackageName, InstallApp, MinimumVersion, IOSBundleId, DynamicLinkDomain	Configure how the email sign-in link is built (redirect URL, platform info, etc.)
SendSignInLinkToEmailAsync	(string email, ActionCodeSettings settings) → Task	Sends a passwordless sign-in email link to the user’s email address.
IsSignInWithEmailLink	(string link) → bool	Checks if the given URL is a valid sign-in-with-email link generated by Firebase.
SignInWithEmailLinkAsync	(string email, string link) → Task<FirebaseUser>	Completes the sign-in flow using the email and the link clicked by the user.
EmailAuthProvider.GetCredentialWithLink	(string email, string link) → Credential	Creates a credential object for linking or reauthenticating a user with email link credentials.

Firebase Product(s)

No response

Targeted Platform(s)

No response