Raft::HTTP::AdminClient
Client side of the /raft/admin/* wire format. Keeps request shapes in
one place so embedders don't hand-roll them.
Single attempt per call, no retry loop — embedders own retry policy.
Returns the response ::HTTP::Status so callers can distinguish
rejection (400, e.g. already a member — stop retrying) from
unavailability (5xx — retry later). Raises only on connection errors.
Class methods
POST <uri>/raft/admin/add_server/<node_id> with a JSON body
{"address": address}. Basic auth is taken from the URI's userinfo
when present.