[MM-25645] api4/user: add migrate_auth endpoints (#14966)

* api4/user: add migrate_auth endpoints

* api4/user: reflect review comments

* add translations

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

i18n/en.json

@@ -75,6 +75,10 @@
     "id": "api.admin.get_brand_image.storage.app_error",
     "translation": "Image storage is not configured."
   },
+  {
+    "id": "api.admin.ldap.not_available.app_error",
+    "translation": "LDAP is not available."
+  },
   {
     "id": "api.admin.remove_certificate.delete.app_error",
     "translation": "An error occurred while deleting the certificate."
@@ -1548,6 +1552,10 @@
     "id": "api.marshal_error",
     "translation": "marshal error"
   },
+  {
+    "id": "api.migrate_to_saml.error",
+    "translation": "Unable to migrate SAML."
+  },
   {
     "id": "api.oauth.allow_oauth.redirect_callback.app_error",
     "translation": "invalid_request: Supplied redirect_uri did not match registered callback_url."