Research cron observer

[ethancjackson]

Changes to cron client that enables conditional processing/propagation of messages, i.e. should_process field. Enables the creation of Observer agents that can have conditional logic for when to propagate a message to process_message for further processing. Defined an PriceChangeObserver agent under examples as well as adding a TokenPriceChangeCalculator tool under alphaswarm proper. This tool wraps the AlchemyPriceHistoryByAddress and gives a more convenient interface for getting price changes directly, as opposed to getting the price history and needing to process it.

[ethancjackson]

The main thing I might want to discuss is:
Should we keep cron_job.py as-is and create a new client to add logic? For example we could call this conditional_cron_job.py instead.
Another point is that I'd like to also create something like a swarm_cron_job.py that allows the user to specify something like a chain of agents to handle processing, so we could have, e.g. PriceForecaster -> TradeAssistant