purefb_server: add local_directory_service and modernize version gate

[ananthakrishnanvk]

SUMMARY

Adds local_directory_service to purefb_server and modernizes the module's REST API version gate.

Key changes:

• New option local_directory_service (str) - Wired into both the create path (ServerPost) and the update path (Server patch). The update diff reads the current value via getattr(server_info, "local_directory_service", None).name and only patches when it differs.
• add_server refactored - the old 2^N if/elif matrix (dns × directory_service) is replaced by a small server_kwargs dict that composes cleanly with the new third option. Behavior is unchanged for the two existing options; the new option slots in without adding another branch.
• API version gate now uses LooseVersion

ISSUE TYPE

• Feature Pull Request

COMPONENT NAME

purefb_server

ADDITIONAL INFORMATION

• Changelog fragment included: 579_purefb_server_local_directory_service.yaml.