Add support for S3-based message for SQS

[nguyenmv2]

Is there a plan for adding supports for SQS messages coming from S3 bucket notification?
The structure of the message is different from a typical SQS message ( I've attached an example below )

{
    "Records": [
        {
            "eventVersion": "2.1",
            "eventSource": "aws:s3",
            "awsRegion": "<some region>",
            "eventTime": "2019-02-22T19:39:33.390Z",
            "eventName": "ObjectCreated:Put",
            "userIdentity": {
                "principalId": ""
            },
            "requestParameters": {
                "sourceIPAddress": ""
            },
            "responseElements": {
                "x-amz-request-id": "",
                "x-amz-id-2": ""
            },
            "s3": {
                "s3SchemaVersion": "1.0",
                "configurationId": "<some configuration id>",
                "bucket": {
                    "name": "<bucket>",
                    "ownerIdentity": {
                        "principalId": "<principalId>"
                    },
                    "arn": "arn:aws:s3:::<bucket>"
                },
                "object": {
                    "key": "test/foo.txt",
                    "size": 1016,
                    "eTag": "",
                    "sequencer": "005C704FF558BE1340"
                }
            }
        }
    ]
}

I would love to contribute a PR. However, my solution assumes all messages always come in this format. I wonder if there's a way to dynamically unmarshal structs in Go.

[bmoffatt]

Sample looks to be the same as events.S3Event https://github.com/aws/aws-lambda-go/blob/master/events/s3.go#L9-L23. Let me know if I've misunderstood the request.