Allow Metadata Configuration in ChannelTemplateSpec

**Problem**
ChannelTemplateSpec only allows setting of the spec for the Channel, but in some cases it may be useful to set some metadata for the Channel as well.

My specific use case is with the [KafkaChannel](https://knative.dev/docs/admin/eventing/kafka-channel-configuration/), which has the `eventing.knative.dev/scope` annotation to change where the Kafka dispatcher is run (cluster/namespace).

**[Persona:](https://github.com/knative/eventing/blob/main/docs/personas.md)**
System Integrator

**Exit Criteria**
A ChannelTemplateSpec can configure metadata of the Channel created by a Broker.