Add returnDID policy

example/returnDID/1.0/policy.rego

+package example.returnDID
+
+_ = {
+    "@context": ["https://www.w3.org/ns/did/v1", "https://w3id.org/security/suites/ed25519-2020/v1"],
+    "id": issuer().did,
+    "verificationMethod": keys.getAll()
+}
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