Add support for using an input JSON file

[atc0005]

Potential scenarios:

1. Use the input file as an attachment array entry.

{
    "type": "message",
    "attachments": [
        {
            // input file content here
        }
    ]
}

2. Use the input file as-is.

[atc0005]

Using this feature, we would effectively have the equivalent of:

#!/bin/bash

webhook_url="https://outlook.office365.com/webhook/a1269812-6d10-44b1-abc5-b84f93580ba0@9e7b80c7-d1eb-4b52-8582-76f921e416d9/IncomingWebhook/3fdd6767bae44ac58e5995547d66a4e4/f332c8d9-3397-4ac5-957b-b8e3fc465a8c"

curl \
    -X POST \
    -H "Content-type: application/json" \
    -d @/tmp/teams-message.json \
    "${webhook_url}"

I borrowed the sample webhook connector URL for illustration purposes.