app wasm & mount relative URLs

[amitu]

If a app is mounted via fastn.app, the wasm files in that app are automatically mounted as well on the same path, e.g., foo.wasm in app mounted at /app/ will be available at /app/foo/.

We will also pass /app/ as X-FASTN-APP-MOUNTPOINT header when any requests to /app/foo/* is passed to foo.wasm.

ftd.mount-relative(url="/asd/") & ft_sdk::mount_relative("/asd/")

We will create a ftd built in function and a ft_sdk function to convert a relative url to full url, eg if app is mounted on /app/ the above will return /app/asd/.

[amitu]

Closing this discussion as duplicate of #2051.