code: public Principal authenticate(List<String> authHeader) throws AuthException {
          if (isAuthEnabled()) {
              String credentials = parseCredentials(authHeader);
              return pravegaAuthManager.authenticate(credentials);
          }
          return null;
      }
docs: Authenticates the subject represented by the specified HTTP Authorization Header value.
  
  @param authHeader contents of an HTTP Authorization header
  @return a {@code principal} representing the identity of the subject if auth is enabled; otherwise {@code null}
  @throws AuthException if authentication fails