Add GoVPP integration and GetVPPInfo gRPC call

VPP client (internal/vpp/)
- New package managing connections to both VPP API and stats sockets,
  treated as a unit: if either drops, both are torn down and
  re-established together.
- Run() loop: connect, fetch version via vpe.ShowVersion, read
  /sys/boottime from the stats segment, log vpp-connect, then monitor
  with control_ping every 10s. On failure, disconnect both, retry
  after 5s.
- Registers as client name "vpp-maglev" (visible in VPP's
  "show api clients").
- Flags: --vpp-api-addr (default /run/vpp/api.sock) and
  --vpp-stats-addr (default /run/vpp/stats.sock). Empty api addr
  disables VPP integration entirely.

gRPC / proto
- Add GetVPPInfo RPC returning VPPInfo: version, build_date,
  build_directory, pid, boottime_ns, connecttime_ns. Both times are
  unix timestamps in nanoseconds — the client computes durations
  locally for display.
- Returns codes.Unavailable if VPP is disabled or not connected.

maglevc
- Add 'show vpp info' command displaying version, build-date,
  build-dir, vpp-pid, vpp-boottime (with duration), and connected
  time (with duration).

internal/grpcapi/server_test.go

@@ -62,7 +62,7 @@ func startTestServer(t *testing.T, ctx context.Context, c *checker.Checker) (Mag
 		t.Fatalf("listen: %v", err)
 	}
 	srv := grpc.NewServer()
-	RegisterMaglevServer(srv, NewServer(ctx, c, nil, ""))
+	RegisterMaglevServer(srv, NewServer(ctx, c, nil, "", nil))
 	go srv.Serve(lis) //nolint:errcheck
 
 	conn, err := grpc.NewClient(lis.Addr().String(),