[TEST] Expose ability to provide http headers when sending requests in our REST tests

`ElasticsearchRestTests` has now a `restClientSettings` method that can be overriden to provide headers as settings (similarly to what we do with transport client). Those headers will be sent together with every REST requests within the tests.
Is there a specific reason the first two args need to be reversed?  I would tend to keep them the same, but I'm not against it changing if there is some kind of justification.