Use rego builtin function to retrieve issuer DID

@yordan.kinkov @valerii.kalashnikov

example/returnDID/1.0/policy.rego

@@ -2,6 +2,6 @@ package example.returnDID
 
 _ = {
     "@context": ["https://www.w3.org/ns/did/v1", "https://w3id.org/security/suites/ed25519-2020/v1"],
-    "id":"did:web:gaiax.vereign.com:policy:policy:example:returnDID:1.0:evaluation",
+    "id": issuer().did,
     "verificationMethod": keys.getAll()
-}
\ No newline at end of file
+}