Add retention mode and legal hold mode on list objects api

[cesnietor]

Fixes https://github.com/minio/console/issues/300
Adds extra fields for list objects api with retention mode, legalhold and isDeleteMarker
For objects with versions needs a version flag on the api:
GET /api/v1/buckets/bucket2/objects?prefix=photo_2020-06-16_13-15-05.jpg&with_versions=true:

{
    "objects": [
        {
            "is_latest": true,
            "last_modified": "2020-10-02 22:51:15.316 +0000 UTC",
            "legal_hold_status": "ON",
            "name": "photo_2020-06-16_13-15-05.jpg",
            "size": 103614,
            "version_id": "314ab4ab-913f-4b32-a706-3b76d7203699"
        },
        {
            "is_delete_marker": true,
            "last_modified": "2020-10-02 22:49:55.826 +0000 UTC",
            "name": "photo_2020-06-16_13-15-05.jpg",
            "version_id": "0156a9ac-8129-44c4-870d-c742b3b7b981"
        },
        {
            "last_modified": "2020-10-02 20:35:46.239 +0000 UTC",
            "name": "photo_2020-06-16_13-15-05.jpg",
            "retention_mode": "GOVERNANCE",
            "retention_until_date": "2020-10-07 22:46:41 +0000 UTC",
            "size": 103614,
            "version_id": "e088545a-d27f-4939-9165-568a0df2eba7"
        },
        {
            "is_delete_marker": true,
            "last_modified": "2020-10-02 20:35:18.911 +0000 UTC",
            "name": "photo_2020-06-16_13-15-05.jpg",
            "version_id": "73ba7b01-d36f-4fa6-997f-0cdfe60fadc8"
        }
    ],
    "total": 4
}

• Tests added