**Is your feature request related to a problem? Please describe.** [Firebase Dynamic Links will stop working August 25th 2025](https://firebase.google.com/support/dynamic-links-faq). [Migration instructions indicate that to use a custom domain one should update usage of `ActionCodeSettings` to use `linkDomain`](https://firebase.google.com/docs/auth/ios/email-link-migration#reuse-custom-dynamic-links-domain), see also in the [Firebase Admin docs](https://firebase.google.com/docs/auth/admin/email-action-links#initialize-actioncodesettings). We currently depend upon `firebase-admin-python` `generate_sign_in_with_email_link` to generate email links, [`firebase-admin-python` hasn't been updated with `link_domain` and currently only supports `dynamic_link_domain`](https://github.com/firebase/firebase-admin-python/blob/master/firebase_admin/_user_mgt.py#L492), [`firebase-admin-node` has been updated with support for `linkDomain`](https://github.com/firebase/firebase-admin-node/blob/master/src/auth/action-code-settings-builder.ts#L109) so I assume this should be officially supported, please let me know if this is not the case. **Describe the solution you'd like** [`ActionCodeSettings` should be updated to accept `link_domain`](https://github.com/firebase/firebase-admin-python/blob/master/firebase_admin/_user_mgt.py#L496) and [`encode_action_code_settings` should be updated to handle `link_domain`](https://github.com/firebase/firebase-admin-python/blob/master/firebase_admin/_user_mgt.py#L531-L536).