Allow Kafka brokers to have associated triggers in other namespaces

**Problem**
Currently broker and trigger need to be in the same namespace. I'd like to be able to have a trigger in a different from the broker.

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

**Exit Criteria**
A measurable (binary) test that would indicate that the problem has been resolved.

Broker:

```yaml
apiVersion: eventing.knative.dev/v1
kind: Broker
metadata:
annotations:
eventing.knative.dev/broker.class: Whatever
name: a-broker
namespace: broker-ns
# other values...
```

Trigger:

```yaml
apiVersion: eventing.knative.dev/v1
kind: Trigger
metadata:
name: a-trigger
namespace: consumer-ns
spec:
broker: a-broker.broker-ns
# other values...
```

This does not work (in the trigger, adding the namespace to the broker name):

```diff
spec:
- broker: a-broker
+ broker: a-broker.broker-ns
# other values...
```

**Time Estimate (optional):**
How many developer-days do you think this may take to resolve?
No idea.

**Additional context (optional)**
Add any other context about the feature request here.

This has been asked by other people, see related: #6577 #5139
I'd ask someone from the team to make sure this does not get stale again, please add a label?

This would be interesting for the Kafka broker, where we can have a centralized one. [See docs](https://knative.dev/docs/eventing/brokers/broker-types/kafka-broker/) for more information.

Start by reading docs/personas.md and the Kafka broker documentation linked in the issue, then review related issues #6577 and #5139. Define the binary test for a Trigger in consumer-ns referencing a Broker in broker-ns, and confirm that this cross-namespace configuration works for the Kafka broker.

Written by the indexing model from the issue text.