Add proper mirror support (#74)

* Add rewrite support and fix existing tests

* Add unit tests for policy matching

* Compile regexes up front and store policies in map

* Add test for verify flow with mirror

* Rename ImageName -> ResolvedName

And only set it when necessary

* Rename Rewrite -> Replacement

but keep it as rewrite in the yaml

pkg/policy/testdata/mock-tuf-wrong-key/mapping.yaml

@@ -2,10 +2,10 @@
 version: v1
 kind: policy-mapping
 policies:
-  - origin:
-      domain: docker.io
-      prefix: library/
-    id: docker-official-images
+  - id: docker-official-images
     description: Docker Official Images
     files:
       - path: doi/policy.rego
+rules:
+  - pattern: "^docker[.]io/library/(.*)$"
+    policy-id: docker-official-images