Allow configuring all drivers via frontend (#1169)

This includes an API endpoint for fetching the list of possible
drivers and their configuration fields. In the future, this can
be extended to include more meta information about each field.

src/Mail/SendmailDriver.php

@@ -17,6 +17,11 @@ use Swift_Transport;
 
 class SendmailDriver implements DriverInterface
 {
+    public function availableSettings(): array
+    {
+        return [];
+    }
+
     public function buildTransport(SettingsRepositoryInterface $settings): Swift_Transport
     {
         return new Swift_SendmailTransport;