Introduce Optable demo cases with Prebid

[ValentinPostindustria]

Closes #30.

Due to Prebid using OpenRTB 2.5, eids are placed in user.ext.eids rather than user.eids as defined in OpenRTB 2.6. Therefore, the current implementation sends the field in user.ext.eids.

    "user": {
        "ext": {
            "eids": [
                {
                    "source": "optable.com",
                    "uids": [
                        {
                            "id": "2065d9e9",
                            "atype": 1
                        },
                        {
                            "id": "15f30409",
                            "atype": 1
                        }
                    ]
                }
            ]
        }
    },