Add finding evidence ingestion

What does this MR do and why?

This MR adds an evidence ingestion task to the security report ingestion service.

Screenshots or screen recordings

postgres.ai plan: https://postgres.ai/console/gitlab/gitlab-production-tunnel-pg12/sessions/8276/commands/29248

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #343279