Introduce serialzeBodyToJson property on HTTP event files

[basvandriel]

Describe your idea/feature/enhancement

Normally, when configuring an FunctionUrl for a Lambda function, you would invoke it like below.

sam local invoke SomeFunction --event some-event.json

Where the event file includes a HTTP payload, containing a raw JSON string for the body.

{
    "version": "2.0",
    "routeKey": "$default",
    "rawPath": "/somepath",
    "rawQueryString": "",
    "headers": {
        "Content-Type": "application/json"
    },
    "requestContext": {
        "http": {
            "method": "POST",
            "path": "/",
            "protocol": "HTTP/1.1"
        },
        "routeKey": "$default",
        "stage": "$default"
    },
    "body": "{\"message\": \"hi\"}",
    "isBase64Encoded": false
}

When the application grows, I would like the application be easier to test locally. Including JSON objects in the body to send data, with a flag to turn this off or on would therefor be valueable.

Proposal

Introduce an optional serialzeBodyToJson property on the object and default it to false. When true, it will directly send the body to the lambda function as a dictionary.