feat(proxy): add support for oci manifest images

[thijmv]

No description provided.

[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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