Allow `onRequest` callbacks to access build-time file content

Related to #29.

This is what works now:

const { EleventyServerlessBundlerPlugin } = require("@11ty/eleventy");

module.exports = function(eleventyConfig) {
  eleventyConfig.setServerOptions({
    middleware: [function overrideMiddleware(req, res, next) {
      // This works now
      res.writeHead(200, {"content-type": "text/html"}); // content-type header required for live-reload
      res.write("Before Text");
      res.end();

      next();
    }],
  });
});

However, I’d like to see folks able to consume res.statusCode (404 when no file is present) and res.body with the static content to do request-time content transforms

Start by tracing how onRequest callbacks and the middleware shown in the issue receive the request and response. Check the behavior for an existing file and a missing file, then verify that callbacks can read static content through res.body and distinguish missing files through res.statusCode (404).