Cloudflare Adapter + Prerendered Function + SSR resolution attempt

[alecbakholdin]

Resolves #15035

This PR is an attempt at hopefully guiding someone in the right direction. I made a best effort to put forward a solution to the issue but ultimately I'm not super confident of the approach here and I'm not knowledgeable enough about the codebase to come up with a better one at this time. Hopefully, someone who knows this area better can take this and run with it, or at least provide some suggestions on how I can improve this more.

This solution solves the problem for cloudflare adapters and locally, but could break other implementations.

Brief Restatement of Problem

when using cloudflare, and likely other adapters, Cloudflare Workers prevents us from making full fetch requests to the same worker, presumably to prevent untraceable infinite call loops? Instead, we're meant to use service bindings like env.ASSETS.fetch().

Solution

1. Replaced the prerendered remote function's fetch call with $app/server's read. (there was already a TODO to allow adapters to inject their own methods here, I thought this might be a step in that direction)
2. Since prerendered routes are stored as files, I added an exception in the $app/server read function to allow prerendered routes as well as server assets found in the manifest

Problems

1. I need help understanding what kinds of implications this could have. It feels like the first change is pretty isolated, tested that CSR and SSR both work now, but I don't know how this plays with other adapters. Help!
2. We don't store the prerendered assets' content lengths, which means that I'm not passing a Content-Length header in the response of the read function. I tried to add the prerendered asset to the manifest._.server_assets object but couldn't quite get there. Help!

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