feat: add support for optional external auth providers (#12021)

docs/api/templates.md

@@ -2002,6 +2002,7 @@ curl -X GET http://coder-server:8080/api/v2/templateversions/{templateversion}/e
     "display_icon": "string",
     "display_name": "string",
     "id": "string",
+    "optional": true,
     "type": "string"
   }
 ]
@@ -2025,6 +2026,7 @@ Status Code **200**
 | `» display_icon`     | string  | false    |              |             |
 | `» display_name`     | string  | false    |              |             |
 | `» id`               | string  | false    |              |             |
+| `» optional`         | boolean | false    |              |             |
 | `» type`             | string  | false    |              |             |
 
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