add check for valid "did:" DIDs

src/libs/endorserServer.ts

@@ -241,6 +241,10 @@ export type CreateAndSubmitClaimResult = SuccessResult | ErrorResult;
 // See https://github.com/trentlarson/endorser-ch/blob/0cb626f803028e7d9c67f095858a9fc8542e3dbd/server/api/services/util.js#L6
 const HIDDEN_DID = "did:none:HIDDEN";
 
+export function isDid(did: string) {
+  return did.startsWith("did:");
+}
+
 export function isHiddenDid(did: string) {
   return did === HIDDEN_DID;
 }