feat: add custom links dropdown, closes #520

[felixmosh]

closes #274

This feature allows to add customLinks to the board via uiConfig object

createBullBoard({
    queues: [new BullMQAdapter(exampleBullMq)],
    serverAdapter,
    options: {
      uiConfig: {
        miscLinks: [{ text: 'Logout', url: '/logout' }],
      },
    },
  });