fix: WebsocketListener nil panic and OIDC auth data race (#5204)

- pkg/util/net/websocket.go: store ln parameter in struct to prevent
  nil pointer panic when Addr() is called
- pkg/auth/oidc.go: replace unsynchronized []string with map + RWMutex
  for subjectsFromLogin to fix data race across concurrent connections

pkg/util/net/websocket.go

@@ -26,6 +26,7 @@ type WebsocketListener struct {
 // ln: tcp listener for websocket connections
 func NewWebsocketListener(ln net.Listener) (wl *WebsocketListener) {
 	wl = &WebsocketListener{
+		ln:       ln,
 		acceptCh: make(chan net.Conn),
 	}