code: public AuthorityKeyIdentifierExtension getAuthKeyIdExtension()
          throws IOException {
          Object obj = getExtension(PKIXExtensions.AuthorityKey_Id);
          return (AuthorityKeyIdentifierExtension)obj;
      }
docs: return the AuthorityKeyIdentifierExtension, if any.
  
  @returns AuthorityKeyIdentifierExtension or null (if no such extension)
  @throws IOException on error