[release-0.21] Add producer interceptor PartitionKeyExtensionInterceptor

[knative-prow-robot]

This is an automated cherry-pick of #751

Add producer interceptor `io.cloudevents.kafka.PartitionKeyExtensionInterceptor` to provide ordered delivery based on the partitioning extension of the CloudEvents spec.

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[slinkydeveloper]

/lgtm

[knative-prow-robot]

[APPROVALNOTIFIER] This PR is APPROVED

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[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (release-0.21@b2aa942). Click here to learn what that means.
The diff coverage is n/a.

@@               Coverage Diff               @@
##             release-0.21     #754   +/-   ##
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  Coverage                ?   73.62%           
  Complexity              ?      436           
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  Files                   ?       77           
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  Branches                ?      132           
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  Hits                    ?     2133           
  Misses                  ?      620           
  Partials                ?      144           

Flag	Coverage Δ	Complexity Δ
java-unittests	80.20% <0.00%> (?)	0.00% <0.00%> (?%)

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[pierDipi]

@googlebot I consent.

[slinkydeveloper]

I've started the dot release job, it should show up in a matter of minutes