adding s390x CI jobs for knative extensions for release 1.15

[dilipgb]

This PR updates the s390x specific CI jobs for knative extensions components kafka-broker and plugin-event.

[knative-prow]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[dilipgb]

@upodroid can you please review the PR.

[dilipgb]

@upodroid client and operator tests are fixed now after community files updated by automation to point to multiarch images (images were missing after chainguard removed aarch, s390x, ppc64le support).

I've raised 1 PR which will fix issues with eventing tests. knative/eventing#8130

I'm looking into kn-plugin-event, meanwhile can you please help in reviewing this PR?

[dprotaso]

Closing see:
#344 (comment)