Refactor: Remove use of ApiPromise to read storage, use storage read via squid instead

[bvotteler]

Two util methods, getForeignAsset and cacheForeignAssets, create an ApiPromise to get foreign asset data from the parachain. We can replace that to use the squid context instead to fetch the same data, keeping it consistent with the rest of our squid implementation.

[bvotteler]

Closed: Cannot as easily get metadata with foreign asset ids using squids built-in storage read mechanisms.