WABP: send media message

[nbartels]

Sending a media message is solved here, hopefully we manage to add everything needed for the different media types (audio, image, document and so on). If this becomes to complex, we have to split the issue according to the media types.

See: https://developers.facebook.com/docs/whatsapp/cloud-api/reference/messages#media-object

{
  "messaging_product": "whatsapp",
  "recipient_type": "individual",
  "to": "PHONE-NUMBER",
  "type": "image",
  "image": {
    "id" : "MEDIA-OBJECT-ID"
  }
}