feature: allow setting custom HTTP response headers and status code in scripts

[vzhemchuzhnikov]

Note: this is only relevant for sync executions.

Currently when script executes, either through webhook or http route, we cannot control the headers and status codes that a client will receive. All responses are sent with 200 status code and the json body.

I propose to add more control to developers, so we could change the response as we need. For example, we could use it to set cookies, redirect and so on.

Ideally, it could be implemented with a special object for every programming language (except shells), which, if being returned, would be converted to a native response for the framework that the API itself uses. However, I understand that could be too much work and will make adding a new language support harder.

As a simpler alternative, it could be implemented just by adding a pre-defined key into the response object, that the API would interpret when returning the response. This could cause collisions with existing responses, so we could name it something like _windmill_http_response_metadata (it would be even better if we could change the name in the settings).
So, to customize the response, a script should:

1. Return an object (dict, struct, etc...)
2. That will contain a pre-defined key

For example, if a python script returns a dict

{
    "_windmill_http_response_metadata": {
        "status_code": 201,
        "headers": {
            "set-cookies": "<your cookies>"
        }
    },
    "message": "Hello world"
}

API should send an http response with the 201 code and set-cookies headers, keeping the default application/json content type and the body, if called through webhook or http route.

[HugoCasa]

it's already possible for status code: https://www.windmill.dev/docs/script_editor/custom_response_code
We can add something similar for headers

[vzhemchuzhnikov]

@HugoCasa cool! I somehow missed the status code. If you could add the same thing for headers, that'd be really great!