Support more WinterCG-like runtimes (export actual fetch-like API)

[verydanny]

I really love the functions, but they're quite limited. I understand a lot is for the sake of performance, but flexibility would be nice too.

For example, I'd love to create a static site adapter for Astro to use with open-runtime functions.

The issue is it's still a bit limited because the open-runtime export default async ({ req, res, log, error }) isn't "web standard". The req, res.send, res.json are not really standard. They're super stubbed-down versions.

Or what if I wanted to implement easy routing using a micro-backend framework like Elysia or Hono?

Scenario:

• I want to create a code documentation website
• I generate the static site with Astro
• I use openruntime-adapter (WInterCG-compatible)
• Serve static site files on a function-based runtime
• Allow custom routes that even accept buffer, etc.

I don't think this is possible without a lot of tinkering with the current API.

[verydanny]

In addition, the limit for formData should be customizable. I'd love for my users to upload things through functions. Right now anything over 1mb causes issues.