feat(BA-942): Add accelerator quantum size field to GQL scaling group

[fregataa]

resolves #3936 (BA-942)

How to test

1. Run this command to inject data before test the API.
   backend.ai mgr etcd put "config/plugins/accelerator/cuda/quantum_size" 1.0

2. new GQL Query

query ScalingGroup {
    accessible_scaling_groups(
        project_id: "PROJECT-ID"
    ) {
        name
        is_active
        accelerator_quantum_size
    }
}

Checklist: (if applicable)

• Milestone metadata specifying the target backport version
• Mention to the original issue
• API server-client counterparts (e.g., manager API -> client SDK)

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📚 Documentation preview 📚: https://sorna--3940.org.readthedocs.build/en/3940/

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📚 Documentation preview 📚: https://sorna-ko--3940.org.readthedocs.build/ko/3940/

[HyeockJinKim]

Is this PR done? @fregataa

[yomybaby]

User can not use scaling_group and scaling_groups. I got an error when I try these using an user account.

{
    "data": {
        "scaling_group": null
    },
    "errors": [
        {
            "message": "Forbidden operation. (superadmin privilege required)",
            "locations": [
                {
                    "line": 2,
                    "column": 3
                }
            ],
            "path": [
                "scaling_group"
            ]
        }
    ]
}

[yomybaby]

LGTM