Draft: Add the principal vulnerability researcher position

Why is this change being made?

Add the principal position to the Vulnerability Researcher so that there is a similar career path for vulnerability researchers to staff, similar as there is for developers. The role description took the equivalent for developers and customized it for the unique skills/background of vulnerability researchers.

Review app: https://vr-add-principal-position.about.gitlab-review.app/job-families/engineering/vulnerability-research-engineer/index.html#principal-vulnerability-research-engineer

Author Checklist

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