Make market_picking decision modular

[gabrielfior]

Similar structure to BettingStrategy.

@ABC
class MarketPickingStrategy:
    def sort_markets(markets: list[AgentMarket]) -> list[AgentMarket]

With this, we could have following strategies:

1. (Standard) Pick markets closing soonest (and some other criteria from verify_market)
2. Pick markets that have little volume (to increase Omen's market coverage) - use cheaper models like gpt3.5
   ...

Also, strategy 2 above should be deployed as a new agent.

[evangriffiths]

@kongzii says "Some long-range markets are being created, but not getting betted on at all by our agents, until they are near closing"