Add Scorecard CI workflow

[AdamKorcz]

This PR adds the Scorecard action to Katibs’s CI.

Scorecard is a tool that scans for a number of supply-chain threats and assesses whether Katib mitigates these threats. Running Scorecard allows projects to avoid degradation in its development pipeline, ie. the action continuously monitors for all Scorecards heuristics and alerts if a given PR increases the risk of a given threat. Projects can also add a badge to demonstrate their Scorecard score.

I highly recommend enabling the “Branch-Protection” check which is disabled in this PR, however, for instructions on enabling it, see the workflow.

Checklist:

• Docs included if any changes are user facing

[google-oss-prow]

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