Finish out http / car-mirror (for non IPFS) fetches.

[zeeshanlakhani]

Summary

Right now, we only fetch Wasm modules from local IPFS resources/URLs or set IPFS gateway. We should leverage car-mirror-rust.

Components

• Integrate car-mirror over HTTP (probably high-level APIs / reqwest-client to start) for non IPFS links.
• Fetching should probably move into background work (can do this after first integration w/ the current in-parallel fetch that happens as part of (local) scheduler initialization.
• Look into sans-io if we want to wrap our own I/O, though the former is probably the best first run.