Implement HTTP proxy for server calls

Even though this can be implemented in parallel, pay attention to who is working on MEN-5011, since you will need to coordinate.

Acceptance criteria:

• The client binds to a random port on localhost interface (should be above 10000, just to be sure)
• DBus endpoints GetJwtToken and JwtTokenStateChange return http://localhost:<PORT>.
• On this port, the client serves HTTP requests (not HTTPS), which are forwarded to the currently authenticated Mender server.
  • Except for the protocol and host, all request and response data is preserved exactly as is in the forwarded call, including headers.
• The code for this needs to be in the github.com/mendersoftware/mender/authmanager package. See MEN-5011.
  • If this package has not been created yet, it is enough that the code is "ready for it" (put in its own file, ready to move; try not to use Mender client internals)
• The client itself does not need to connect to this endpoint yet (will be handled implicitly in MEN-5011 by switching over to call the DBus endpoints).
• Unit tests
• Feature branch: feature-unified_comms