Draft: Add code_flows property to vulnerability details

What does this MR do?

Adds an allowed details field - code_flows, consisting of a nested array of code_flow_node. Each array describes a detected flow.
This field will be used to hold cross-function cross-file results with source -> propagation -> propagation... -> propagation -> sink like flows.

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab/-/issues/455559+s
https://gitlab.com/gitlab-org/security-products/security-report-schemas/-/issues/21+s

Checklist

• Ensure changes can be built upon without requiring a breaking change, see Building for Extensibility.
• Review and add/update tests for this feature/bug.
• Add an entry to the CHANGELOG if required, with the appropriate version. See Classifying Changes.
• Assign the MR to the appropriate person/people for review.