Goal: 
Implement application credential support in the Swift backend driver
to enable zero-downtime password rotation for Glance when using Swift
as image storage backend with service user authentication.

Currently, the Swift backend uses `V3Password` authentication for
trustee authentication in multi-tenant mode, which causes backend
operations to fail when service user passwords are rotated. This
story adds support for `V3ApplicationCredential` authentication,
allowing seamless credential rotation without service disruption.
Acceptance Criteria:

• Patch should be ready for upstream review