feat/trigger: enable trigger to send to different namespace

[zhongduo]

After this change, when the namepsace is specified in the subscriber
the URI can be resolved correctly to the specified namespace instead
of forcing it to the be same as the trigger namespace.

Fixes #2015

Proposed Changes

• Don't over the namespace of a trigger

• 🎁 Allow triggers to send to a sink in different namespace

Release Note

- 🎁 Allow triggers to send to a sink in different namespace

Docs

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[zhongduo]

/assign @Harwayne

[tayarani]

/lgtm

[knative-metrics-robot]

The following is the coverage report on the affected files.
Say /test pull-google-knative-gcp-go-coverage to re-run this coverage report

File	Old Coverage	New Coverage	Delta
pkg/reconciler/trigger/trigger.go	67.1%	66.4%	-0.7

[liu-cong]

/lgtm

BTW, can you test this in a cluster? Just to confirm this isn't something else that blocks cross-namespace. I remember there used to be some logic in eventing webhook.

[zhongduo]

/lgtm

BTW, can you test this in a cluster? Just to confirm this isn't something else that blocks cross-namespace. I remember there used to be some logic in eventing webhook.

I am actively testing it now with my experiment to see if cross namespace delivery is possible. After this change, I can now send from curl pod in default namespace to broker/trigger in cloud-run-events-example namespace, then got routed back to event-display default namespace.