FEATURE: Add email_encoded parameter to accept inbound base64 encoded emails (#12947)

We have found when receiving and posting inbound emails to the handle_mail route, it is better to POST the payload as a base64 encoded string to avoid strange encoding issues. This introduces a new param of `email_encoded` and maintains the legacy param of email, showing a deprecation warning. Eventually the old param of `email` will be dropped and the new one `email_encoded` will be the only way to handle_mail.

lib/auth/default_current_user_provider.rb

@@ -49,7 +49,7 @@ class Auth::DefaultCurrentUserProvider
       methods: :post,
       actions: "admin/email#handle_mail",
       formats: nil
-    )
+    ),
   ]
 
   # do all current user initialization here