Async-customerio

* [FIX] Authentication data is not sent with request.

* Refactored ``async_customerio.client_base.AsyncClientBase`` to take advantage of connection pool. So the HTTP client will be created once during class ``async_customerio.track.AsyncCustomerIO`` and ``async_customerio.api.AsyncAPIClient`` instantiation.

- Support of Python 3.12 added.
- [FIX] Add missing dependency `type_extensions` to be able using type annotations on Python 3.7

- Added support of sending transactional push notifications. More details of how to send Push Notifications via CustomerIO <https://customer.io/docs/journeys/transactional-api/#transactional-push-notifications>

- Added helper function for validation of request origin using X-CIO-Signature and X-CIO-Timestamps

- [BREAKING] For consistency across APIs following actions have been taken:
- Renamed transactional email request optional arguments `amp_body` to `body_amp` and `plaintext_body` to `body_plain`
- Changed transactional email request default values for optional boolean arguments `disable_css_preproceessing`, `queue_draft`, `disable_message_retention`, `tracked`, `send_to_unsubscribed`