feat: Swift service should support Keystone v3 authentication

[benroeder]

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