FEATURE: Allow users to sign in using LinkedIn OpenID Connect (#26281)

LinkedIn has grandfathered its old OAuth2 provider. This can only be used by existing apps. New apps have to use the new OIDC provider.

This PR adds a linkedin_oidc provider to core. This will exist alongside the discourse-linkedin-auth plugin, which will be kept for those still using the deprecated provider.

lib/validators/linkedin_oidc_credentials_validator.rb

@@ -0,0 +1,23 @@
+# frozen_string_literal: true
+
+class LinkedinOidcCredentialsValidator
+  def initialize(opts = {})
+    @opts = opts
+  end
+
+  def valid_value?(val)
+    return true if val == "f"
+    return false if credentials_missing?
+    true
+  end
+
+  def error_message
+    I18n.t("site_settings.errors.linkedin_oidc_credentials") if credentials_missing?
+  end
+
+  private
+
+  def credentials_missing?
+    SiteSetting.linkedin_oidc_client_id.blank? || SiteSetting.linkedin_oidc_client_secret.blank?
+  end
+end