Requires a signed-in user, and uses the PKCE flow.
Enable manual linking in your project before using this method.
The call returns a URL. Redirect the user to it to authorize the new identity.
Examples
Link an identity
var state = await supabase.Auth.LinkIdentity(Provider.Github, new SignInOptions());
// Redirect the user to authorize the linked provider.
var authorizeUrl = state.Uri;