Add publish-test-images job to the registry-cache extension

[ialidzhikov]

What this PR does / why we need it:
This PR adds publish-test-images job to the registry-cache extension.
The final goal is extension developer to be able to publish images from PR by running /test pull-gardener-extension-registry-cache-publish-test-images

Which issue(s) this PR fixes:
Part of gardener/gardener-extension-registry-cache#3

Special notes for your reviewer:
N/A

[ialidzhikov]

/kind enhancement

[oliver-goetz]

/lgtm
/approve

[gardener-prow]

LGTM label has been added.

Git tree hash: ab179a25d04168ce006a46c3efaea934ae9a6f35

[gardener-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: oliver-goetz

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[gardener-prow]

@ialidzhikov: Updated the job-config configmap in namespace prow at cluster gardener-prow-trusted using the following files:

• key gardener-extension-registry-cache-test-builds.yaml using file config/jobs/gardener-extension-registry-cache/gardener-extension-registry-cache-test-builds.yaml

In response to this:

What this PR does / why we need it:
This PR adds publish-test-images job to the registry-cache extension.
The final goal is extension developer to be able to publish images from PR by running /test pull-gardener-extension-registry-cache-publish-test-images

Which issue(s) this PR fixes:
Part of gardener/gardener-extension-registry-cache#3

Special notes for your reviewer:
N/A

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