add currency endpoint #360

we would like to be able to switch currencies and the frontend wants to pull exchange rates from the api. We need to query and store exchange rate for an external provider and offer a query endpoint

https://github.com/balancer/frontend-v3/blob/main/lib/shared/utils/currencies.ts#L31-L43

we return the full list in the query
add eth and btc price to the list (at query time, not to the DB, so it stays up to date)
re-query every 10mins to stay in request limits