[Trigger - Spec] Ordered events support

This issue is to discuss whether First In First Out (FIFO) or ordered events should be something that's configurable through the spec.
The prominent implementations of Broker/Trigger currently already support ordered events through annotations:

Kafka: By setting kafka.eventing.knative.dev/delivery.order to ordered or unordered
Rabbitmq: FIFO vs high throughput is controlled by the same annotation rabbitmq.eventing.knative.dev/parallelism. When == 1, events will be in order.

Ordered events can be an important part of an application's setup and using annotations to configure it requires users to dig through the implementers documentation.

Open questions:

• Should supporting ordered events be a requirement for implementations or be left as a MAY support?
• Where in the spec should the ordered field be placed? One suggestion during the working group meeting was in the DeliverySpec
• Should the default be ordered events?

Start by reviewing the Trigger spec, especially the proposed DeliverySpec location, and compare the existing Kafka and RabbitMQ annotation behavior described in the issue. Done means resolving whether ordered events are required or optional, deciding where the ordered field belongs, and defining the default; no specific file or test is named.