feat(monitor): add basic-auth feature for protected sites

[ejsadiarin]

this closes issue #316

This feature properly resolves sites that require basic auth credentials, so that it properly returns status OK instead of Unauthorized.

At the very least, this can replace using alt-status-codes and adding 401 to it, which is like a "bogus" status OK (although if the intent is to just ping the site if its alive, then its good enough of a solution).

- type: monitor
  cache: 1m
  title: Services
  sites:
    - title: Traefik
      url: https://traefik.example.com
      icon: di:traefik
      basic-auth:
        username: your-username
        password: your-password

Furthermore, this could be expanded to also pass the configured basic auth credentials to the request when the user clicks on the specific monitor widget.

[svilenmarkov]

Thanks for contributing!