🔧 Allow links in needservice directive

[filipepcampos]

Hi! I noticed that it was not possible to create needs with links (custom or otherwise) through a custom service.

Looking a bit deeper into the existing open-needs service, it seems like it should support links
e.g. from the tests:

    {
        "key": "NEP_003",
        "type": "Requirement",
        "title": "Rocket painting",
        "description": "Red and blue. No other colors please.",
        "format": "txt",
        "project_id": 1,
        "options": {
            "status": "open",
            "priority": "low",
            "costs": 20000,
            "approved": 1,
        },
        "references": {"links": ["NEP_001", "NEP_002"]},
    },

Just added the link keys to the allowed options, similarly to what's done in the need.py directive

[ubmarco]

This looks functionally ok. Thanks for the fix.
Just one concern.