Add BoundedContext abstraction

[carlos-verdes]

A single cqrs framework should be able to manage more than one aggregate.

Related with #90

[davegarred]

That's certainly not the design here.

This is probably more of a naming problem since 'framework' implies you might handle multiple aggregates.

[carlos-verdes]

Correct me if I'm wrong but let's say I need to implement a Query that read events from 2 different aggregates, from the documentation the simplest Query example looks like this:

struct SimpleLoggingQuery {}

#[async_trait]
impl Query<BankAccount> for SimpleLoggingQuery {
    async fn dispatch(&self, aggregate_id: &str, events: &[EventEnvelope<BankAccount>]) {
        for event in events {
            println!("{}-{}\n{:#?}", aggregate_id, event.sequence, &event.payload);
        }
    }
}

On this example (and others looks similar) I have to provide one aggregate type only, hence I can't handle events from other one.

One option is to create an enum with all aggregates the query is interested in + implement From trait like this:

enum BankQueryInput {
  case BankAccount(BankAccount),
  case TaxesAggregate(TaxesAggregate),
}

impl From<BankAccount> for BankQueryInput {
  ...
}

This way I could write a Query which is interested on bank account movements and tax events.
Let me know if this example has sense to you.