update show_unit_extensions api to DRF

[awais786]

verify via postman
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form-data

url = block-v1:edx+cs222+23111+type@sequential+block@14a256f0a4e44466978ec41fca103710

expected result

{
    "header": [
        "Username",
        "Full Name",
        "Extended Due Date"
    ],
    "title": "Users with due date extensions for Subsection",
    "data": [
        {
            "Username": "admin",
            "Full Name": "aaaa",
            "Extended Due Date": "2024-08-29 01:30"
        },
        {
            "Username": "newserviceuser",
            "Full Name": "awab",
            "Extended Due Date": "2025-11-11 00:00"
        }
    ]
}

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Verify via Instructor Dashboard
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