cluster-bootstrap should support setting auth headers in client requests

[pjfanning]

• issue came up during discussions about [chore] improve error handling when invoking 'start' twice #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 cluster-bootstrap support TLS requests in client calls #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