Enable Account Linking for Existing Users with GitHub

[miicolas]

When a user attempts to log in with GitHub using an email address already associated with an existing account created via credentials (email and password), the server should offer an option to link the GitHub account to the existing account. This feature will ensure a seamless user experience and prevent the creation of duplicate accounts.

Steps to Reproduce:

Create an account using an email and password.
Log out and try to log in with GitHub using the same email address.
Expected Behavior:
The server should detect that the email address is already associated with an existing account and prompt the user to link the GitHub account to the existing account. Upon successful linking, the user should be able to log in with either credentials or GitHub.

Actual Behavior:

The server redirects the user to an error page, indicating that the account was created with credentials, and does not provide an option to link the GitHub account.