Update FilerEtcStore to use a function that returns the current active
filer instead of a fixed address, enabling high availability.
Changes:
- Add SetFilerAddressFunc() method to FilerEtcStore
- Store uses filerAddressFunc instead of fixed filerGrpcAddress
- withFilerClient() calls the function to get current active filer
- Keep SetFilerClient() for backward compatibility (marked deprecated)
- Update S3ApiServer to pass FilerClient.GetCurrentFiler to store
Benefits:
- Credential store now uses currently active/healthy filer
- Automatic failover when filer becomes unavailable
- True HA for credential operations
- Backward compatible with old SetFilerClient interface
This addresses the credential store limitation - no longer pinned to
first filer, uses FilerClient's tracked current active filer.