Organize topic name builder function and usages

[aliok]

Fixes #

Proposed Changes

• Organize topic name builder function and usages
• Move the channel specific one to kafka package. Needed for future testing.
• Hardcode topic name format for KafkaSink testing

Release Note

Docs

[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/kafka/topic.go	100.0%	97.5%	-2.5

[codecov]

Codecov Report

Merging #1416 (9c1642e) into main (c108f41) will decrease coverage by 0.02%.
The diff coverage is 66.66%.

@@             Coverage Diff              @@
##               main    #1416      +/-   ##
============================================
- Coverage     76.24%   76.22%   -0.03%     
  Complexity      584      584              
============================================
  Files            98       98              
  Lines          3658     3659       +1     
  Branches        166      166              
============================================
  Hits           2789     2789              
- Misses          653      654       +1     
  Partials        216      216              

Flag	Coverage Δ
java-unittests	82.28% <ø> (ø)

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Impacted Files	Coverage Δ
control-plane/pkg/reconciler/kafka/topic.go	97.67% <0.00%> (-2.33%)	⬇️
control-plane/pkg/reconciler/broker/broker.go	70.44% <100.00%> (ø)

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[pierDipi]

Nice, thanks!

/lgtm
/approve

[knative-prow-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: aliok, pierDipi

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[aliok]

/test pull-knative-sandbox-eventing-kafka-broker-integration-tests