[release-0.25] Add namespace to referenced addressable before resolving destination

[knative-prow-robot]

This is an automated cherry-pick of #1254

The fields `Broker.spec.delivery.deadLetterSink.ref.namespace` and `Trigger.spec.delivery.deadLetterSink.ref.namespace` are optional.
By default the controller uses the object namespace.

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[pierDipi]

@googlebot I consent.

[google-cla]

All (the pull request submitter and all commit authors) CLAs are signed, but one or more commits were authored or co-authored by someone other than the pull request submitter.

We need to confirm that all authors are ok with their commits being contributed to this project. Please have them confirm that by leaving a comment that contains only @googlebot I consent. in this pull request.

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[knative-metrics-robot]

The following is the coverage report on the affected files.
Say /test pull-knative-sandbox-eventing-kafka-broker-go-coverage to re-run this coverage report

File	Old Coverage	New Coverage	Delta
control-plane/pkg/reconciler/broker/broker.go	85.8%	86.2%	0.4

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (release-0.25@bf35b7e). Click here to learn what that means.
The diff coverage is n/a.

@@               Coverage Diff               @@
##             release-0.25    #1262   +/-   ##
===============================================
  Coverage                ?   75.82%           
  Complexity              ?      460           
===============================================
  Files                   ?       87           
  Lines                   ?     3036           
  Branches                ?      136           
===============================================
  Hits                    ?     2302           
  Misses                  ?      568           
  Partials                ?      166           

Flag	Coverage Δ
java-unittests	80.04% <0.00%> (?)

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[matzew]

/retest

[pierDipi]

/retest

[knative-prow-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: knative-prow-robot, pierDipi

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