Implement basic authentication

[ziccardi]

Currently only keycloak authentication is implemented.

It should be added here: https://github.com/aerogear/unifiedpush-admin-client/blob/master/src/UnifiedPushAdminClient.ts#L62

[stale]

Is this still relevant? If so, what is blocking it? Is there anything you can do to help move it forward?

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs.