chore: validate images from registry via Pepr (ADR)

[btlghrants]

Description

Working through how we might accomplish validating image signatures against pubkeys derived from the OCI registries that serve them.

Relates to #1240

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Other (security config, docs update, etc)

Checklist before merging

• Unit, Journey, E2E Tests, docs, adr added or updated as needed
• Contributor Guide Steps followed

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.82%. Comparing base (b11d6ca) to head (5e2bb0e).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1256   +/-   ##
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  Coverage   85.82%   85.82%           
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  Files          29       29           
  Lines        1411     1411           
  Branches      329      329           
=======================================
  Hits         1211     1211           
  Misses        182      182           
  Partials       18       18