Support custom price, supply and backing query code per asset

[Spenhouet]

The current system expects a generic / standardized way how price, supply and backing is derived. In many cases this seems to not work. For example prices for token contracts might not be tracked on DefiLlama or DexScreener but could be derived from an oracle. Or backing might work differently than the standard of having a balance in a contract.

We want to add the capability of supporting any custom logic to derive price, supply and backing for an asset via custom query code per asset which is executed if provided. This will allow to properly cover more assets.