Add optional systemd service to launch spdm_emu_responder

[Jino26]

This is to propose adding an optional systemd service file to allow spdm_emu_responder to be launched and supervised on systems with systemd support, so that it can be utilised as a long-running responder and provide more granular controls.

Proposed location to add service file:
spdm_emu/spdm_responder_emu/spdm-emu-responder.service
This keeps the service definition close to the source code it manages.

Proposed service behavior (high level)

• Type: simple
• ExecStart: launch spdm_emu_responder with configurable arguments
• Restart policy: on-failure
• Logging: via journald (no custom log handling)
• arguments to spdm_responder_emu can be passed via environment file if required
• Not enabled by default (opt-in only)

If this approach is acceptable, happy to submit a follow-up PR with the service file and documentation if any.

[steven-bellock]

@Jino26 that all sounds reasonable.