runtimes/js: Added middleware functionality

[fredr]

Adds a way to set custom status codes and an api for passing data from middlewares to the handler.

const mw = middleware(async (req, next) => {
  // add data that can be read from the handler
  req.data.myMiddlewareData = { some: "data" };

  const resp = await next(req);

  // change the status code on the response
  resp.status = 418;

  return resp
});

To access data in the handler

export const get = api(
  { expose: true, method: "GET", path: "/hello/:name" },
  async ({ name }: { name: string }): Promise<Response> => {
    const callMeta = currentRequest() as APICallMeta;
    const myData = callMeta.middlewareData?.myMiddlewareData;

    return { message: "Hello " + name };
  },
);

[encore-cla]

All committers have signed the CLA.

[fredr]

I think this should be fine, as there shouldn't be any contention on this lock, but alternatively we can add a function that fetches this data on demand instead