Begin architecture document for tracking vulnerabilities across multiple branches

Why is this change being made?

Vulnerability Management and Tracking across multiple branches is potentially one of the most requested Sec features in GitLab and has been for a long time. For a variety of reasons it has not yet been feasible to implement this due to scaling and performance concerns.

Finally after a concerted effort to improve a variety of these concerns, none-the-least the decomposition of the Sec database to give us dedicated DB room to facilitate such substantial features, we should now be able to consider implementing a feature like that which will require tracking orders of magnitude more information than was previously necessary, and holding a similarly massive amount of data ready in cache so that users can filter and read it on demand.

While the new Sec DB gives us a lot of room, we still need to be sure that what we build here will scale and function well lest we commit ourselves to years more technical debt.

This MR served as a springboard for a lot of brainstorming to find a suitable implementation that will allow us to track vulnerability differentials across a codebase and construct vulnerability reads to view a particular branch on demand.

Author and Reviewer Checklist

Please verify the check list and ensure to tick them off before the MR is merged.

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Commits

• Begin architecture document for tracking vulnerabilities across multiple branches

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