chore: validate images from registry via Pepr (impl) (#1262)

## 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)
- [x] Other (security config, docs update, etc)

## Checklist before merging
- [x] Unit,
[Journey](https://github.com/defenseunicorns/pepr/tree/main/journey),
[E2E Tests](https://github.com/defenseunicorns/pepr-excellent-examples),
[docs](https://github.com/defenseunicorns/pepr/tree/main/docs),
[adr](https://github.com/defenseunicorns/pepr/tree/main/adr) added or
updated as needed
- [x] [Contributor Guide
Steps](https://docs.pepr.dev/main/contribute/#submitting-a-pull-request)
followed

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Co-authored-by: Case Wylie <[email protected]>