Allow UI config translation for more fields

Problem/Motivation

The current schema definition for the config prevents fields such as the login,logout redirect URLs along with the SP entity ID values to be translated in the config translation UI.

Steps to reproduce

Go to the existing samlauth translation page and it only lists the menu link titles.

Proposed resolution

Change the config schema to use "label" instead of "string" for those fields. According to https://www.drupal.org/docs/drupal-apis/configuration-api/configuration-... →

translatable: Whether the defined type is translatable; Note: not all types can be made translatable. To make a type: string translatable, change it to type: label instead of adding translatable: true, or it will not show up in translation forms.