[NEW] Add MMS support to Voxtelesys

[john08burke]

Hello!

I have updated the Voxtelesys SMS provider to include support for inbound / outbound MMS messaging.

Full API docs here.

In short, inbound callbacks have the following structure:

{
  "type": "mo",
  "to": "00000000000",
  "from": "00000000000",
  "body": "message text here",
  "media": [
    "https://smsapi.voxtelesys.net/api/v1/media/{file_id}.{file_ext}?token={token_value}"
  ],
  "received_at": "2020-01-01T00:00:00Z"
}

Outbound messages have the following structure:

{
  "to": ["00000000000"],
  "from": "00000000000",
  "body": "message text here",
  "media": ["{media_URL}"]
}

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