Async-firebase

* [FIX] The push notification could not be sent to topic because ``messages.Message.token`` is declared as required attribute though it should be optional.
``messages.Message.token`` turned into Optional attribute.

* The limit on the number of messages (>= 500) that can be sent using the ``send_all`` method has been restored.

Remastering client interface
* [BREAKING] The methods ``push`` and ``push_multicast`` renamed to ``send`` and ``send_multicast`` accordingly.
* [BREAKING] The signatures of the methods ``send`` and ``send_multicast`` have been changed.
* Method ``send`` accepts instance of ``messages.Message`` and returns ``messages.FCMBatchResponse``
* Method ``send_multicast`` accepts instance of ``messages.MulticastMessage`` and returns ``messages.FCMBatchResponse``
* New method ``send_all`` to send messages in a single batch has been added. It takes a list of ``messages.Message`` instances and returns ``messages.FCMBatchResponse``.
* ``README.md`` has been updated to highlight different in interfaces for versions prior **3.x** and after
* Improved naming:
* ``messages.FcmPushMulticastResponse`` to ``messages.FCMBatchResponse``
* ``messages.FcmPushResponse`` to ``messages.FCMResponse``
* ``utils.FcmReponseType`` to ``utils.FCMResponseType``
* ``utils.FcmResponseHandler`` to ``utils.FCMResponseHandlerBase``
* ``utils.FcmPushResponseHandler`` to ``utils.FCMResponseHandler``
* ``utils.FcmPushMulticastResponseHandler`` to ``utils.FCMBatchResponseHandler``
* Type annotations and doc string were updated according to new naming.

* Class ``AsyncFirebaseClient`` has been refactored. Communication related code extracted into base class.

* ``async_firebase.encoders.aps_encoder`` no longer clears ``custom_data`` dictionary, as this causes subsequent notifications to not get any content in ``custom_data`` dictionary.

* Add object for sending Web Push.