Add public authentication() method to Artifactory backend

[hagenw]

In order to deprecate the audfactory package we should add the private function audbackend.core.artifactory._authentication() as a public class method.

username, password = audbackend.Artifactory.authentication(host)

This would a user allow to test which username and password would be used to authenticate with the host,
useful for debugging authentication issues, see https://audeering.github.io/audfactory/usage.html#authentication.
And it would allow to get the username, password needed for rest API requests, as in audfactory.rest_api_get():

    username, apikey = authentification(url)
    return requests.get(url, auth=(username, apikey))

[frankenjoe]

Implemented in #215