Added support for FSM in Forum topics.

The strategy can be changed in dispatcher:

.. code-block:: python

    from aiogram.fsm.strategy import FSMStrategy
    ...
    dispatcher = Dispatcher(
        fsm_strategy=FSMStrategy.USER_IN_TOPIC,
        storage=...,  # Any persistent storage
    )

.. note::

    If you have implemented you own storages you should extend record key generation
    with new one attribute - `thread_id`
