Build queries from more than 1 aggregate

[carlos-verdes]

If we apply CQRS the commands and queries have different models, and it will be super normal to consume more than one Aggregate event for a single Query.

Simplified example for trading platform:

• aggregate to manage user portfolio --> allow user to track assets, events like AssetDeposited, AssetWithdrawn, AssetBought, AssetSold
• aggregate to update prices --> it will register all assets from all users portfolios and every hour will get last price
• portfolio overview query --> combining the balance of assets of a user (using events from first aggregate) and the update in price (using second aggregate) will show total usd value per portfolio/asset to final user

Related with #94