Owns a gRPC h2 listener, supervised under livery_grpc_server_sup.

The h2 listen socket is owned by whichever process calls h2:start_server, so it must be a long-lived one or the socket closes with it. This gen_server is that owner: it opens the listener in init and holds it for the lifetime of the supervised child, so a server started from a transient caller (a test's init_per_suite, a short request) keeps running.