ci: Add Scorecard workflow

[AdamKorcz]

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

Scorecard is a tool that scans for a number of supply-chain threats and assesses whether Notebook 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.

[google-oss-prow]

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[andyatmiami]

We will need to rework this PR a bit to account for the fact that actual feature development occurs on a couple non-standard branches:

• notebooks-v1
• notebooks-v2

It is also important to note that the notebooks-v1 and notebooks-v2 branches themselves are wholly unrelated/mutually exclusive/contain completely different code bases.