Add basicauth support to gatsby-source-drupal

[gatis]

I have a business case where we need to build gatsby page from drupal webiste that is protected by basicauth (Basic access authentication). Since axios already supports it I slightly modified the gatsby-source-drupal code to enable it. Please see if you can implement this feature as proposed in this PR or similar manner.

[pieh]

I think this make sense - only thing is that

auth: {
  username: 'thisismyvery',
  password: 'simplepassword'
}

might seem like these are credentials to drupal admin - might be worth to use more descriptive fields like htaccess_username or rename auth to basicAuth? Another request would be to not put plain string in your example of usage - please replace them with process.env.BASIC_AUTH_USERNAME or something like this (we don't want to "encourage" users of using secrets as plain text).

[pieh]

Thanks @gatis!

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