js: add with-sec-fprs to getKeysArmored

• Reflects the changes made to gpgme-json in commit 6cc842c9aa76d19448141e5117ac59452d7a1ff3.
  • getKeysArmored now returns an object with property 'armored' being the exported armored block, and an (optional) array of fingerprint strings for those keys that can be used in sign/encrypt operations as property 'secret_fprs'. With this, extensions such as mailvelope will be able to bulk fetch all necessary key information in one request.