feat(proxy): add support for oci manifest images

[thijmv]

This PR (1/3) adds OCI manifest support to Kraken. OCI manifests are the modern Docker standard and enable layer compression with additional algorithms (e.g. zstd), which may improve transfer and decompression times.

This PR makes the following changes:

• OCI parsing
  • Add parsing support for OCI manifests.
  • Peek at the media type of manifests, rather than trying each possible format.
• Prefetching
  • Fix dependency resolution for prefetching, which excluded (sub-)manifest and config blobs.
  • Return an error when sub-manifest blobs cannot be downloaded, instead of silently continuing with a partial dependency list.

Fixes #574.

[Copilot]

Pull request overview

Adds OCI image manifest/index support across Kraken’s Docker manifest parsing, proxy prefetch/preheat paths, puller utility, fixtures, and CI coverage.

Changes:

• Extends manifest parsing to support Docker v2 manifests/lists and OCI manifests/indexes through a shared dockerutil.ParseManifest.
• Updates proxy prefetch/preheat and puller behavior to consume OCI manifests and include manifest/config blobs where applicable.
• Adds OCI fixtures/tests and runs integration tests with containerd snapshotter enabled and disabled.

Reviewed changes

Copilot reviewed 8 out of 9 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
utils/dockerutil/dockerutil.go	Adds OCI-aware manifest parsing and supported Accept media types.
utils/dockerutil/dockerutil_test.go	Covers Docker and OCI manifest parsing cases.
utils/dockerutil/fixtures.go	Adds OCI manifest and index test fixtures.
proxy/proxyserver/prefetch.go	Uses shared parser and descends manifest lists/indexes for blob collection.
proxy/proxyserver/preheat.go	Allows OCI image manifest push notifications.
proxy/proxyserver/server_test.go	Updates and adds prefetch tests for manifest lists and OCI indexes.
tools/bin/puller/pull.go	Uses shared parser and requests OCI single-arch manifests.
.github/workflows/build-and-test.yaml	Adds integration matrix for Docker and OCI/containerd snapshotter modes.
test/python/conftest.py	Removes stale OCI-support TODO comment.

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