WABP: Support for incoming image message

[nbartels]

{
  "object": "whatsapp_business_account",
  "entry": [
    {
      "id": "101809759215303",
      "changes": [
        {
          "value": {
            "messaging_product": "whatsapp",
            "metadata": {
              "display_phone_number": "123456789",
              "phone_number_id": "106342955420583"
            },
            "contacts": [
              {
                "profile": {
                  "name": "TestUser"
                },
                "wa_id": "49123456789"
              }
            ],
            "messages": [
              {
                "from": "49123456789",
                "id": "wamid.HBgNNDkxNTIwOTgzMzM5OBDGEJFEYFDNBM0Q2NEExMzkxRUNDRDA1NkNCAA==",
                "timestamp": "1653316648",
                "type": "image",
                "image": {
                  "caption": "Some awesome image",
                  "mime_type": "image\\/jpeg",
                  "sha256": "law0CgE277wMMnIJU0XIxhDne6Ptmwaek\\/thVM7mVtg=",
                  "id": "400962571939895"
                }
              }
            ]
          },
          "field": "messages"
        }
      ]
    }
  ]
}