feat: Swift service should support Keystone v3 authentication

The Swift backend only supports pre-obtained static tokens. Users must manually acquire a token (e.g. via curl) and pass it in config. Tokens expire, requiring manual refresh.

Swift deployments commonly use OpenStack Keystone v3 for authentication. The backend should support native Keystone v3 password authentication with automatic token acquisition and refresh.

This would also allow automatic discovery of the Swift storage URL from the Keystone service catalog, so users don't need to know the exact endpoint.

Reference: https://docs.openstack.org/keystone/latest/api_curl_examples.html

Start at the Swift backend entry point and consult the linked Keystone v3 API examples for password authentication and service-catalog responses. Done means users can authenticate without pre-obtaining tokens, tokens refresh automatically, and the Swift endpoint can be discovered from the catalog.

Written by the indexing model from the issue text.