Add function for parsing DOI definition files (#172)

Add a Rego builtin called `attest.internals.parse_library_definition`
for parsing the DOI definition files in
https://github.com/docker-library/official-images/tree/master/library.
This will allow us to verify DOI provenance fields against these files
which are the source of truth for DOI images.

This function just defers to
https://github.com/docker-library/bashbrew/blob/master/manifest/rfc2822.go.

policy/testdata/policies/test/def_parse/def_parse_test.rego

@@ -0,0 +1,18 @@
+package def_parse_test
+
+import rego.v1
+
+test_parse_library_definition if {
+	def := `Maintainers: me <me@example.com> (@me)
+GitRepo: blah
+
+Tags: 1, 2, 3
+GitCommit: fa105cb3c26c8f0e87d7dbb1bf5293691ac2f688
+File: Dockerfile.foo`
+	result := attest.internals.parse_library_definition(def)
+	definition := result.value
+	definition.Entries[0].GitRepo == "blah"
+	definition.Entries[0].GitCommit == "fa105cb3c26c8f0e87d7dbb1bf5293691ac2f688"
+	definition.Entries[0].Tags == ["1", "2", "3"]
+	definition.Entries[0].File == "Dockerfile.foo"
+}

policy/testdata/policies/test/fetch/fetch_test.rego

@@ -0,0 +1,9 @@
+package attest_test
+
+import rego.v1
+
+import data.attest
+
+test_sucess if {
+	attest.success
+}

policy/testdata/policies/test/fetch_test.rego

@@ -1,7 +0,0 @@
-package attest
-
-import rego.v1
-
-test_sucess if {
-	success
-}