Document http authentication for metricbeat http module

[ppf2]

#4268 is closed, but authentication for the http module is not yet documented.

This can be specified via a headers map with a base64 string (example below is for elastic:changeme) for basic authorization, eg.

...
...
  request.enabled: true
  response.enabled: true
  headers:
    Authorization: Basic ZWxhc3RpYzpjaGFuZ2VtZQ==