Create asset type config for selectors

Based on this note https://github.com/sense-finance/defi-rates-crawler/pull/1#discussion_r720507078 in the review done in PR https://github.com/sense-finance/defi-rates-crawler/pull/1.


Pull each asset type out into a config, so adding or removing assets doesn't require a recompilation. Consider using the `config` crate: https://docs.rs/config/0.11.0/config/.