Contributes to https://gitlab.com/gitlab-org/gitlab/-/issues/199716

**Problem**

We store static_objects_external_storage_auth_token values in
plaintext.

**Solution**

* Fill static_objects_external_storage_auth_token_encrypted fields
* Use optional (encrypted first, plaintext is a fallback) strategy
* Store encrypted version of the token when user updates it

Changelog: added