cluster-bootstrap should support setting auth headers in client requests

* issue came up during discussions about #418
* if someone sets up basic auth on their pekko management endpoints then cluster-bootstrap needs to be able to call those endpoints
* related to but separate from #426
* the question is if we should allow users to supply the auth settings statically via config
* or if we should strive to derive the auth settings based on incoming HTTP requests and echo those in outgoing requests - and how this might even be achieved

Start by reading the discussion in #418 and the related issue #426, then trace the cluster-bootstrap calls to Pekko Management HTTP endpoints. The issue leaves open whether authentication should come from static configuration or incoming requests; done requires an agreed approach that lets configured basic-auth endpoints be called successfully.

Written by the indexing model from the issue text.