Add basic authentication authentication for plugin manager

The plugin downloader which is invoked by the plugin command line does not support downloading the plugin from a URL which is secured using basic auth.

Use Case
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In the continuous integration process if we want to install a plugin where the plugin artifact is available on the CI server which is restricted with basic auth this is not possible.

Workaround
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Download using wget passing auth details and use the elasticsearch plugin command line passing the *file* url instead of *http* url

Good to have
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Plugin command line to have options to take in basic auth credentials and pass it on to the downloader.

Thanks to @katta for the original PR!

Closes #2550.
Given the way that `AuthCredentials` are currently used, you perform a `null` check where it matters and this class is effectively unnecessary as-is.