Add Google Chat webhook support

[przemoch-dev]

Description

We are not using slack, and sending alert notification to channel other than slack, without using proxy tools is currently impossible.

I would love to have Google chat webhook integration, but the current JSON format includes fields that are forbidden in the Google chat webhook API.

{
  "error": {
    "code": 400,
    "message": "Invalid JSON payload received. Unknown name \"username\" at 'message': Cannot find field.\nInvalid JSON payload received. Unknown name \"attachments\" at 'message': Cannot find field.",
    "status": "INVALID_ARGUMENT",
    "details": [
      {
        "@type": "type.googleapis.com/google.rpc.BadRequest",
        "fieldViolations": [
          {
            "field": "message",
            "description": "Invalid JSON payload received. Unknown name \"username\" at 'message': Cannot find field."
          },
          {
            "field": "message",
            "description": "Invalid JSON payload received. Unknown name \"attachments\" at 'message': Cannot find field."
          }
        ]
      }
    ]
  }
}

It seems to be an easy feature, as it only requires excluding username and attachments fields from the payload. I tested it by sending only the text field via API, and it works as expected with proper message formatting.

I would really appreciate this feature in the future releases.