Lift http handler type discovery up a layer

[alexcrichton]

This commit lifts the discovery of what type of handler a component is, for example spin-vs-wasi-vs-wasi-snapshot, up one layer from where it currently resides. Previously this determination was made on each request and now it's made when a component first loaded and pre-instantiated instead. This would shift possible errors earlier in the system, for example.

This removes a bit of per-request work and makes the creation of the handler in execute_wasi a bit more straightforward with less guessing of exports for something that was already probed.