(XXX Investigate frequency of response errors from Cow XXX) Implement caching mechanism for cow quotes

[gabrielfior]

We are relying on Cowswap as a price oracle.
Recently' it's been failing quite often, so we should cache the prices so that we can reliably price our positions and estimate trade sizes.

Idea for this ticket:
-> Run a CronJob that queries Cow (and handles errors gracefully) and fills the cache (Postgres)
-> db_cache decorator should be used
-> Agent operation also fills the cache
-> Caching price should be "smart", so that price for 1 unit of token X can be derived from price of other amount of token X (do some analysis on Cow UI, but maybe simple mean is good enough)