File processing support

This could allow for downloading a for example compressed file and decompressing it before it is cached like described in [esbuild#936](https://github.com/evanw/esbuild/issues/936#issuecomment-818620960).

It should be rather trivial to implement and could look like this:
```ts
const file = await cache("https://example.com/file.zip", (stream: ReadableStream) => {
return stream.pipeThrough(unzipTransformStream);
});
```