generate-registry-metadata: generate metadata individually for steps

[AlexNPavel]

This PR changes the way that metadata gets generated so that each step
has its own metadata file that gets stored in the same directory as the
step. This allows PRs that update metadata (like OWNERS file updates) to
be approved by the approvers of the individual steps instead of
requiring an approval by the dptp team as was the case with the
monolithic metadata file.

This also removes the separate registry-metadata directory and adds
the OWNERS and metadata files to the main registry directory. This was
done to make sure that the configresolver can load a registry with
metadata, which was not the case when a separate registry directory was
being used. A new make target has been added to verify that the metadata
is up-to-date and be added as a unit test to CI.

[openshift-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: AlexNPavel, stevekuznetsov

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Needs approval from an approver in each of these files:

• OWNERS [AlexNPavel,stevekuznetsov]

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[openshift-bot]

/retest

Please review the full test history for this PR and help us cut down flakes.

[openshift-ci-robot]

@AlexNPavel: The following test failed, say /retest to rerun all failed tests:

Test name	Commit	Details	Rerun command
ci/prow/images	76168c9	link	/test images

Full PR test history. Your PR dashboard.

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