feat: add policy, oci, attestation

pkg/policy/testdata/mock-tuf-allow/doi/data.yaml

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+config:

pkg/policy/testdata/mock-tuf-allow/doi/policy.rego

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+package docker
+
+import rego.v1
+
+allow := true

pkg/policy/testdata/mock-tuf-allow/mapping.yaml

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+# map repos to policies
+version: v1
+kind: policy-mapping
+policies:
+  - origin:
+      domain: docker.io
+      prefix: library/
+    name: docker-official-images
+    description: Docker Official Images
+    location: doi